Stalling when hot

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PJ3
Cucciolo - the Lil Pup
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Stalling when hot

Post by PJ3 »

I recently purchased a 1973 750GT that had been totally restored a few years ago by a well know restorer and then parked. I am new to bevel heads so excuse me if I get some of the terminology wrong. The engine has 9.3 to 1 compression S style pistons and 32mm DelOrto carbs. When I first started it up, while it started first kick on the "choke circuit, the engine was surging suggesting a lean condition or wrong ignition timing. Also the pipes were very blue though I recognize that this could just be the ethanol in the fuel. First thing I did was check the static timing. It was 6 degrees BTDC so I set both cylinders to 10 degrees. It is running Dyna S ignition though it appears to have traditional coils. I also changed out the manifold gaskets with the aluminum ones with O-rings that Steve sells. I also replaced the O-rings on the idle mixture screws and the idle stop screws. I took the carbs apart and found them to be totally clean but I still sprayed carb cleaner throughout. I checked the float levels and was careful not to put any pressure on the needle valve when checking. They were set too low (suggesting inadequate fuel in the bowl for idle) so I adjusted them to 18mm. The idle jet is a "60" in both carbs. After making all these adjustments the bike will still start first kick on the choke circuit and then idle perfectly at just over 1,000 rpms within about 20 seconds and runs extremely good for about twenty minutes. At that point it simply will not idle. I have tried various approaches to setting the mixture screws (base setting of 1.5 turns out) with no success. The engine will either just stop immediately when you come to a stop or it will idle for about ten seconds, then begin to "hunt" and die. The engine will restart and then do the same thing over and over again, but if you ride it the engine is fine beyond idle.

Given the bike runs fine when cold and runs fine when hot off idle I have, maybe incorrectly, ruled out an electrical issue as the core of the problem. My theory is that something is changing the fueling of the engine when it is warm. The only area of a possible air leak that I have not addressed is the clamp that connects the carbs to the manifolds. The fit seems very good for both carbs so this does not seem like the problem but again I have no experience here.

Any and all words of wisdom are welcome.

Thanks,

PJJ3
baddean
Parallel Twin
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Re: Stalling when hot

Post by baddean »

PJ3,
You didn't mention if you have separate levers on each carb or if you have a cable operated lever on the handlebar.
Either way, check to make sure that when you turn of the "choke" (enrichener) off it is all of the way off.
If it is slightly open the bike will run great cold and well on throttle but will be too rich at idle (when hot) possibly causing what you describe.
Basically, check the enricheners to be sure that they are not still "enrichening" when you close them.
Also you didn't mention an air cleaner (or cleaners). Make sure it (they) are clean and moving air.
Hope this helps,
Let us know.
Dean
Faster, faster until the thrill of speed overcomes the fear of death.
Hunter S Thompson RIP
PJ3
Cucciolo - the Lil Pup
Posts: 7
Joined: Thu Jul 11, 2013 7:10 am

Re: Stalling when hot

Post by PJ3 »

Hi Dean:

Thanks for the reply. The bike has a handlebar mounted lever that controls both of the starting or enrichener circuits on the carbs. Your suggestion makes sense and I had not checked that out previously. I just took a look and the lever is shutting down the plugs on the enrichener circuits properly so that is not the problem. The bike has K&N air filters and they are clear so plenty of air is getting in.

Last night I decided to try a bunch of things that seemed pretty crazy but since I had run out of normal ideas I figured I might as well try them. First I was wondering if the pressure was being created in the gas tank as the bike got warm which was limiting the flow of gas. Clearly this would not explain why the bike runs fine off idle when hot but simply won't idle. Anyway, the two vent holes in the gas cap were clear. I then decided to make two more adjustments to the carbs. I replaced the top cover O-rings on both carbs even though the current O-rings were not very old. Next I took out both the mixture screw and the throttle stop screws and stretched their springs out by roughly 1mm to increase the pressure they exert on the new O-rings I had put on each. I know this seems a bit crazy but I was wondering if at high temperature the springs were not exerting enough pressure on the O-rings to keep excess air from entering. I was pretty convinced excess air was the problem since when the bike won't idle the mixture screw seems to have no effect on how the bike is running suggesting there must be a leak somewhere. Unfortunately, I made both of these changes (new top O-rings and stretched the springs) at the same time so I am not sure which had a positive effect but the bike is now much better. I am reluctant to say it is totally fixed since I have thought that before but I rode it about sixty miles and pulled over several times and it would idle perfectly at 1,200 rpms. This seemed pretty miraculous to me so I intend to take it for another ride tomorrow to make sure the fix is permanent. I also ordered new springs from Steve and will see how long those springs are compared to ones that were on my bike. My current theory is that the fellow who had the bike before me probably could never get it to run properly, was constantly fiddling with the carbs and just wore the springs out so they could not hold. The reason he would have done this is that I believe the bike had other air leak issues (intake manifolds) that I fixed before being left with just the hot idle problem. Anyway, that is my theory and I sticking with it until proven wrong.

Thanks again for your reply.

Peter
wdietz186
Cagiva Alazzura
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Re: Stalling when hot

Post by wdietz186 »

Peter, One more thing to check. Look at the bottom of the choke pistons, With time and use the rubber that seals the choke passages can deteriorate and wear so much that the choke circuit will be partially open causing an overrich mixture when the engine is hot. Sometimes they seal and everything seems fine until the next time you use the choke, then it isn't.
PJ3
Cucciolo - the Lil Pup
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Re: Stalling when hot

Post by PJ3 »

Thanks that is something that I had not looked at until now. I did not realize there was a rubber piece at the bottom of the choke pistons. I just took mine out to inspect. The rubber on my choke pistons seems to be inside the bottom of choke piston itself and does not protrude down below the piston lip. Is it supposed to extend down as a seal or is just a plug to prevent air flow thru the inside of the choke piston itself?

Yours and Dean's input has gotten me looking much more closely at the choke circuit and realizing how it can screw up the entire carb operation if it is not set correctly. I have confirmed that both choke pistons are fully extending by insuring there is sufficient play at the lever to allow the choke piston springs to push them down to seal.

As you can probably tell I am new to DelOrto carburetors and they are way more sophisticated than Amals which I am used to..

Thanks,

Peter
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BevHevSteve
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Re: Stalling when hot

Post by BevHevSteve »

Peter - replace those choke pistons and all should be right in the world. The rubber disk on the bottom doesn't protrude but it does rest on a little brass tube. The rubber become s hard over time and ceases to seal up - yes I keep them in stock.
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PJ3
Cucciolo - the Lil Pup
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Re: Stalling when hot

Post by PJ3 »

HI Steve:

As it turns out my problem was definitely not the choke pistons. I rode the bike for 100 miles today and it now idles perfectly. The problem of not idling when hot was definitely due to an air leak that starved the engine for fuel as opposed to getting to much fuel. The problem was either the O-rings at the top of the carbs or the extra tension I put on the mixture and throttle stop springs. The reason I am not sure which was the culprit was that I made both those modifications at the same time. If you an opinion I would love to hear it since I have worked on motorcycles for a long time (not necessarily that well but definitely long) and have never run into exactly this problem. I was starting to think I might have a carb with a hair line crack or some other strange condition.

I really appreciate the input and education I got from you and others about the cold start (or choke) circuit and how problems there can create engine running problems. I will definitely keep an eye on this in the future.

Thanks,

Peter
baddean
Parallel Twin
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Re: Stalling when hot

Post by baddean »

Peter,
That's great news. Glad you worked it out.
If I may... Based on the changes you made together I would say it would have been the o-rings in the carb tops that helped the issue.
The springs on the throttle stop screw and the air screw are there primarily to put tension on them so they will stay where you set them as well as holding the washer and o-ring in place.
When you said that the air screw adjustments made no changes it could have been because you were getting air though the "bad" o-rings in the carb top.
With Amal carbs used on the British bikes and early GTs the slide would wear in the carb bore letting air leak past and cause the exact same thing you were experiencing. With that said, if this comes up again, check the slide to carb bore clearance. The same slide wear could occur in the Dell'Orto.
Now it's time to put some miles on her (him?)
Dean
Faster, faster until the thrill of speed overcomes the fear of death.
Hunter S Thompson RIP
PJ3
Cucciolo - the Lil Pup
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Joined: Thu Jul 11, 2013 7:10 am

Re: Stalling when hot

Post by PJ3 »

Thanks Dean. I suspect you are right about the problem being an air leak from the top. I have had situations with Amals in the past where the mixture jet spring loses tension and the setting moves around. That was not happening with the DelOrtos on my Ducati 750 so the springs were probably fine, but I will measure the new ones I ordered from Steve to see if they are the same length as mine, that is, prior to my stretching them.

I will pay more attention to the clearance between the slides and the carb bodies. I know this is a big problem with Amals and many people either put sleeves in them to reduce wear or just change them out since they are not that expensive. DelOrtos are way more complicated (and I believe expensive) and I have gotten a crash course over the last couple of weeks in some of that complexity.

This is my first Ducati bevel head and it is a stunning engine. I have a 1974 Norton Commando remanufactured by Les Emery of Norvil and before that I had a Kenny Dreer VR880. Of course Norton was using their isolastic system to try to survive given they did not have the money for a new engine whereas Ducati had that new engine. I actually like riding the Ducati 750 better than riding my Ducati ST4s as it is a much more mellow experience due to the greater flywheel effect, though obviously they are not in the same league in terms of performance. I wish I had bought a bevel head years ago

Thanks for your help.

Peter
baddean
Parallel Twin
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Re: Stalling when hot

Post by baddean »

Ha, Peter,
Great minds seem to thing alike. ( and ride similar bikes)
I have a 1968 Norton Commando Fastback. (carbs sleeved)
Like you I prefer the 750GT to it.
When I get to ride my 1996 Ducati 916 it still makes me giggle though. Hell they all make me smile in my helmet.
Be safe out there,
Dean
Faster, faster until the thrill of speed overcomes the fear of death.
Hunter S Thompson RIP
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ColinS
Mariana
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Re: Stalling when hot

Post by ColinS »

Hi Guys, I hope you don't mind me jumping in on the Commando / Ducati bit. I've just sold my '72 750 Combat because the 900 GTS and the Darmah SSD that I have do all that even a modified Commando (twin discs, 920 motor, 5 speed box) will do but without leaking oil. And of course , the sound!! Then for other days I have a rubber band M900 as well.

Cheers

Colin
Colin Steer
Keswick
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