Bridgestone Motorcycle Parts Discussion Board
Bridgestone Tech Talk => 350 Talk => Topic started by: BRT-GTR on July 25, 2017, 01:52:34 PM
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I've finally sorted it but the following may help others with similar engine problems.
When I received my 350 GTO last December she would start and run but I wasn't able to get the tickover settings right. I've spent weeks if not months trying to find the problem. The engine /crank had been rebuilt previously by the man himself, Steve Reed and she was fitted with NOS carbs. Compression is good on both pots.
I've been able to get either cylinder running reasonably well but not both at the same time. When both cylinders were pulling equally she would smoke more on one side or the exhaust note would not sound right, 2 stroking on one and 4 stroking on the other. I'm sure one pot was not fully firing at times (cool muffler). Also got conflicting symptoms for rich or weak mixture, engine was very zingy when accelerated (lean) and smoked (rich) or wouldn't return cleanly to idle. No response whatsoever from adjusting pilot air screws on 30 size standard pilot jets. What was going on ?
Here's what I've done to date with very little change to the idle :-
Cleaned carbs, clearing all passages and reset float height. Needle in middle groove and 140 mains in while running in new pistons and rings.
Installed new points and condensers and reset timing
Cleaned exhausts and baffles
Cleaned and overhauled oil pump, reset stroke. Cleaned check valves
Checked disc valves were installed correctly
Remade various electrical joints to cure suspected weak spark on the left. Fitted separate engine earth strap.
Right cylinder was spitting oil droplets from the pipe. New crank seal in disc cover cured that.
Finally, based on advice on the site, I put in 22.5 pilot jets. This did produce a result and I could now optimise the right cylinder mixture/speed on the pilot system at tickover but the left side still didn't respond, in fact the cylinder would run with the airscrew fully closed. Thinking the pilot jet was blocked, I pulled the left carb yet again, everything was clean. While resetting the pilot airscrew, I noticed the spring was going coil bound when screwed in to the closed position. Didn't see this on the bike because I was usually stood over it.
Whipped the airscrew out to find it had been overtightened, the 2.5mm hole at the bottom was opened out - damn, new carb required ?. Nothing to lose, I was able to press a short length of one eighth od brass tube into the seat to restore it back to size. A bit fiddly but it worked. Now able to optimise the airscrew setting on both pots, balance the cylinders and she now ticks over nicely, pinging on both pots with an even note. The smoke is evenly balanced on both sides and the tailpipes are reasonably dry.
Learned a lot in the process, main things being, you can't tune a dragged cylinder, only the lead cylinder. Raise tickover to 2000 rpm for the pot you want to tune. A dragged or misfiring pot will smoke. The pipes spit water droplets when cold, don't confuse with oil.
I was close to giving up and just running it as it was. The lesson? - never assume new items are OK. Don't know when the overtightening occurred, the airscrew NOS springs don't help being a bit too strong for the job they do. Got it booked in for it's MOT test (annual goverment check) on Friday, so I can get it UK registered and start to enjoy being back on a Bridgestone after 45 years.
Brian , now a happy bunny :D ;D :D
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A good read, good advice and a great result. Best of luck for your MOT.
Hardy
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I agree with hardy.... good catch Brian! 8)
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Great guidance, Brian. I have been pondering how best to clean check valves. How did you do this?
...bert
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Great info, well done, always satisfying to get it right in the end.
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The GTO passed the MOT and I sneaked a couple of extra laps round the block on the way back home. My first ride on two wheels for the best part of 45years and on a BS, what could be better, was 19 again ;D ::)
Should be able to ride it legally in a few more weeks when UK registered. Couple of items to look at, a bit of play in the steering head bearings and the engine vibrated more than I remember it should at 4000 rpm (didn't go over this, new pistons). Any suggestions ?
Bert, As you will know, not a lot you can do with the check valves. Just rinsed them in kerosene, put the banjo back on with an 8mm nut and blew them through, forwards with a foot pump. (Have to be careful how I phrase this) You can check how easily the ball is lifting off it's seat by sucking at the bottom end. You will hear the valve open. Also, thinking the valve might have been allowing oil to drain into the crankcase at rest, I used a short piece of wire to give the ball a very light tap to make sure it was seating fully.
Brian.
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Despite having a little play in the head bearings the steering felt stiff at low speed.
Bit the bullet yesterday and pulled the forks. Found a total of 42 balls in the races, should be 38 !! :o Cleaned and repacked, steering is now as sweet as a nut and I have 4 spare balls for the clutch pushrod - result.
Brian
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Nice job avoiding suggestive phrasing, Brian! Thanks for all these good pointers.
...bert
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....snip.... Whipped the airscrew out to find it had been overtightened, the 2.5mm hole at the bottom was opened out - damn, new carb required ?. Nothing to lose, I was able to press a short length of one eighth od brass tube into the seat to restore it back to size. A bit fiddly but it worked. ....snip....
Hey Brian, I've attached a pic of 2 carbs side by side (left carb upside down ;D ) with the pilot air screws screwed in to the point that they first meet resistance. As you can see, the right carb screw has a couple threads still showing, while the left carb has virtually none.
So, I can only surmise that the left carb pilot air screw has been over tightened.
Now to just toss this out there, the left carb is also a later (after 21H) vs the right carb which is an earlier GTR smaller float needle valve carb. Was there a difference between the 2 in how the passage for the pilot air screw was drilled out and tapped? The screw itself is the same for both carbs. I still think the left has been over tightened but was hoping I might get lucky and find it was just a difference between model years. ::)
Anyway, how difficult was your brass tube repair? What length did you use? And finally.... what do you mean when you say it was a "bit fiddly"?
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Mike,
Can't answer your question re the two carb versions but suspect you are right about the left carb being overtightened. A 2.5mm drill should be a good fit in the hole at the bottom of the air screw drilling. If it wobbles the hole has been expanded.
Bit fiddly = extremely difficult but I'm a sucker for punishment and had nothing to lose (apart from pair of NOS carbs ;D). Pilot screw seat/orifice sits between the two drillings on the front of the carb (Open hole and blanked hole in your photo) so there's only 4mm of metal in which to insert a brass liner. You can't see what you are doing and are working blind, hoping the liner ends up in the right position and doesn't get pushed too far in.
Thought afterwards, if I was to attempt another repair, think I would look for a tubular brass rivet with a bore of 2.5mm, like the ones on the side panel badge. That would have a flanged head and give a positive stop for the insert. A pop rivet might also work.
Note also the open drilling on the front of the carb does not sit on the centre line of the pilot screw, the pilot drilling comes in at the bottom, so drilling out the damaged seat means visually judging at what angle to hold the carb, aaaaaaaaarrrrgh :o
Over to you Mike but you're on your own with this one. Good luck, Brian. (Currently watching the TT superstock race on TV)
PS, I came to the conclusion that with a tickover speed of 1800rpm, the idling engine is very close to coming on to the needle jet circuit. If you find the hole is expanded, put 22.5 pilots in, set the airscrew 11/2 turns out and just run it as it is. Fine tuning the airscrew doesn't really make that much difference.
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I think I'm going to take your advise and just leave them the way they are and run the 22.5 jets.
Just spent 4+ hours experimenting on a couple of trashed out carb bodies I had laying around and couldn't come up with anything I felt comfortable with.
If anybody else has any ideas.... jump in! ;D
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Ok, further research has me back to thinking there might be a difference between the "before 21H" and "after 21H" carbs.
Checkout the 2 photos below.
The first photo is the one where I started with a "after 21H" carb (my GTO carbs) and using a center punch, I marked off the depth to where the screw would begin to seat using a piece of masking tape.
The second photo is a "before 21H" carb (67/68 GTR) with the same center punch used in the first photo. Clearly it doesn't go in as deep as the GTO carbs.
Did something get changed between the end of production for the GTR's and the start of the GTO's? All I know, is that all my early GTR carbs have the shorter depth and all my GTO carbs have the deeper measurement.
I'm leaving them alone! :o
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Mike,
Can you see/measure any difference in the centre to centre dimension of the two carb openings IE, blanked and open drilling.
If not, I suggest the difference is simply due to machine settings at Mikuni when the carbs were produced. If I've got my logic right, the later carbs were drilled deeper (say 1mm+) . Doesn't sound much but with only 4mm of metal behind the airscrew seat it may have weakened the seat resulting in the seat being more susceptable to overtightening damage.
Brian.
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Brian, Checking the center to center dimensions I basically got the same results as when I used the larger diameter center punch, only this time I used a drill bit that was just large enough to keep it out of the center hole. The first picture below shows this with the carb on the left being an "after 21H" and the right one is a "before 21H.
Next, I donned the my strongest magnifying glass headgear and my best flashlight and proceeded to try to get the best view I could of the innards!
The second pic is my best rendition of what I observed. Both have threads all the way down to where the blocked off air passage intersects the screw passage. This is where the difference begins.
In the "after 21H" carb, there is a larger gap/space to where the "floor" begins to slope down to the center opening.
Hopefully when I'm finished doing the cross-section on the "before21H" carb, it'll verify my observation...... at least on one carb. Sorry, I don't have a "damaged" "after21H" to play with. :(
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Success & confirmation..... at least for the "before21H" carb.
Now if sometime in the future my observation for the "after21H" carb is likewise confirmed, we can say with certainty that there was indeed a change in the air flow screw passage, along with the previously known float needle valve change.
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....snip.... If I've got my logic right, the later carbs were drilled deeper (say 1mm+) .
After double checking my measurements, I'd say it's probably closer to 1mm exactly.
The increased depth would make it tougher to bottom out and damage the seating orifice since you'd probably also be close to bottoming out on the outside spring.
Just my thoughts... ;D
EDIT: edited to erase some flawed logic on where I thought the screw tip would seat on the deeper passage. DOH!
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This is a slight hijacking of this thread, but does anyone know the size number of the stock needles? I changed the needle jets back in the 70's on mine, trying to lean out the lower part throttle running, but my notes don't have the stock needle number. I have the stock needle jet number, in fact, still have the stock needle jets. I'd like to go back to stock there and try the 22.5 idle jets now that others have found the issue.
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Karl, I have some nos needles and they have a square boxed symbol followed by 4D3. Hope that helps. ;D
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Thanks very much, Mike, for sectioning the idle circuit, good job,very helpful
What is the length of the 2.5mm section hole. Looks like a shouldered rivet could be used to restore the seat and would make a better job than the brass tube I used. I was worried it might partially blank the the carb drilling that runs back to the pilot jet.
I still feel the differences you are finding between the early/late carbs are just normal production variations. I can't really see any benefits from an engineering point of view. The needle has to be 'bottomed out' to be able to set the 'turn out'. The main problem is the spring strength which makes it difficult to feel when the screw tip just touches the seat and the carb alloy is quite soft, resulting in the orifice being easily enlarged. This makes it difficult, if not impossible to fine tune the idle mixture. My engine idled quite happily with the air screws fully closed.
Another piece in the jigsaw, Brian
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Thanks, that does help, mine are the correct ones. That's one thing I won't have to look for.
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Thanks very much, Mike, for sectioning the idle circuit, good job,very helpful. What is the length of the 2.5mm section hole. Looks like a shouldered rivet could be used to restore the seat and would make a better job than the brass tube I used. I was worried it might partially blank the the carb drilling that runs back to the pilot jet. ...snip....
Overall, it's pretty close to the 4mm you thought it'd be. I can't be real accurate though since this was the carb I was experimenting on with the brass tubing you suggested. So the hole has been drilled out a bit and had several attempts at inserting the brass tubing in there which stretched and marred it up a bit. Sorry....
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....snip.... I still feel the differences you are finding between the early/late carbs are just normal production variations. I can't really see any benefits from an engineering point of view. The needle has to be 'bottomed out' to be able to set the 'turn out'. The main problem is the spring strength which makes it difficult to feel when the screw tip just touches the seat and the carb alloy is quite soft, resulting in the orifice being easily enlarged.
Another piece in the jigsaw, Brian
To me, the "engineering point of view" would be lessening the chance of over-tightening and damaging the seat.
I measured the distance between the top edge of the casting and the top edge of the screw itself first without the spring and fully seated and then with the spring on and screwed in until I felt resistance from either it being seated or the spring bottoming out. The measurements were virtually identical. So with the spring on there's no possibility of bottoming out and damaging the seat and if it wasn't fully seated, it's close enough that when you "turn out", it's probably like you said... "Fine tuning the airscrew doesn't really make that much difference."
I need to get my hands on a "after21H" carb body to cross-section! ;D
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OK, I understand what you are saying re the spring acting as a limit stop on the screw. That would require very accurate machining and forming of all the parts and drillings required. Not impossible but unlikely on a high volume production basis, in my opinion.
At least one of the airscrew seats on my NOS carbs had been previously overtightened to the point where the airscrew was ineffective.
We might have to agree to disagree on this one but again many thanks for the carb section. If I attempt another seat repair, I will post full details.
Brian.
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nice autopsy DR Moonpup
ive never had a problem with this but thanks for the posts
I'm working on a set of newer carbs at the moment
Mike if you have a set of new and old bowls laying around it might be worth a photo
I cant upload a picture from my phone and have lost the camera for my computer
cheers
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Andrew, I don't have any nos bowls, just the ones on the carbs and a couple extra that I've cleaned up & lapped. (see photo below)
Is there something in particular you're looking to verify in regards fuel/float bowls? I'm not aware of any differences for those.
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Here's a couple of float bowl variations that I found on my spare carbs.
First pic, note only single starter circuit boss on right carb - may be from a pre 1000 engine. All my other bowls have two bosses.
Second pic, note strengthening webs on front of right carb bowl - I believe this is a late carb and an attempt to reduce flange warping. There are no webs added to the rear flange.
Brian
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Wow, thanks Brian.... I learned something new this morning! banana
There are at least 3 different bowls. 8)
1- single boss/no webbing
2- double boss/no webbing
3- double boss/webbing
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the later model carb with the two enriching tubes ,one is blanked off at the base from casting .unless mine is blocked with crud it seems to serve no purpose
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Great work Mike on the cross section of the carb!
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Great work Mike on the cross section of the carb!
Thanks..... it was actually kind of fun!
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the later model carb with the two enriching tubes ,one is blanked off at the base from casting .unless mine is blocked with crud it seems to serve no purpose
Yep, that side is non-functioning and serves no purpose for our bikes.