Variable idling 1275 tuned Marina engine

[MMT1968]

When I am back home next week I’ll take a ride and see if covering that small hole suspected to leak AIR into the intake manifold with some JB-weld was a successful attempt. If not I’ll have to look further.
I guess JB-weld can stand the heat at the inlet manifold. If not I‘d have to dismantle the manifold, drill that hole and tap it down to close it with a screw. I hope that is not necessary.

Chris

[Bill_qaz]

I'm sure JB weld with be fine with inlet manifold temperature until an online petro chemical physicist tells us otherwise
Hope it resolves your issue, definitely an interesting puzzle

[MMT1968]

You‘re right, Bill. That’s why I posted this case in this forum. „Online petro chemical physicist“
It’s most likely that a rised idle that can not be adjusted sufficiently is caused by some secondary air. But finding the leak is a puzzle. JB-weld is said to stand 230°c, so it should work - hopefully. It’s hard to believe that a tiny little hole closed by a rivet could cause the problem.
I‘ll post it here if it works or not.

Chris

[Bill_qaz]

Chris, is it just an illusion of your pic or is the front float chamber not perpendicular?

Screenshot_20240606-152102_eBay.jpg (98.28 KiB) Viewed 1785 times

[MMT1968]

Hi Bill,

That‘s an illusion, just by the pic. It is at the right angle.
Thanks for looking!

Chris

[MMT1968]

That tiny hole in the manifold seems to have an influence on the idle revs. Having closed it with some JB-weld reduced the idle speed to 1100revs. But after warming up idle is rising to 1800 revs and after a short push on the accelerator it falls to 1100 revs again. So next step is to close the hose to the brake servo and see what‘s happening.

Chris

[Guildbass]

I had a new motorcycle new in 1999 that had carburettors and consequently they were set incredibly lean at idle for emissions reasons.

The result was.. A steadily rising idle as the engine warmed up, and we're talking over 20 minutes, and horrendous backfiring on the over-run when throttle 'off'

Luckily, the carbs still had idle mixture screws, albeit blanked off, so I adjusted for 'best' idle, backed off the throttle set screws, and got it right.

Your symptoms suggests a very lean idle fuel mixture to me......
I presume the mixtures have been checked?

[MMT1968]

Well, I thought about that also and checked the spark plugs. They all look perfectly light brown so the mixture should be ok. It’s a bit of a puzzle and I have to go one step after the other. Next is closing the hose to the brake servo to see if there could be an air leak.

Chris

[Guildbass]

That tiny hole in the manifold seems to have an influence on the idle revs. Having closed it with some JB-weld reduced the idle speed to 1100revs. But after warming up idle is rising to 1800 revs and after a short push on the accelerator it falls to 1100 revs again. So next step is to close the hose to the brake servo and see what‘s happening.

Chris

I've seen this when one carb is opening fractionally before the other or one carb's idle is higher. As the SU relies on engine vacuum the carb with the higher idle setting holds the revs up and the engine vacuum so generated pulls or holds the second one up until the second one drops it down. Can you check with a Manometer to confirm the same airflow through both at idle?

[MMT1968]

The pistons in the carbs are moving up and down synchronously and close completely so I can only guess that the air flow is equal in both carbs as I don’t have a manometer to hand. But I have an appointment in early November for a dyno test to find the correct needles. The carbs and ignition will be checked and adjusted by then anyway. If I don’t find the reason for the rising idle when warm I hope they‘ll find it then. But I keep searching as the problem bothers me seriously.

Chris

[Bill_qaz]

Chris, when it's idling high have ýou tried manually pushing the throttle closed?
If the return spring is not strong enough then at idle, max depression the engine maybe sucking the butterflies partially open. Also do you carbs have poppet valves in the butterflies?

[MMT1968]

Hi Bill,

No, luckily there are no poppet valves in the butterflies. It doesn’t reduce the idle when I press the accelerator back manually nor is there any further movement in the butterfly axle. I‘ll try to add a second spring but have not much confidence that this will help as the spings I have are all new. If that would be the reason it would not only rise the idle when warm.

Chris

[Bill_qaz]

Definitely sounds as if induction leak, something opening up worse when warm.
Which is what you believed in beginning, carb cleaner and hot engine is dodgy combination.
I have heard of talc being used to indicate induction leak. The only other thing could be a crack in the inlet casting of the head itself.

[MMT1968]

As the head was fully ported it had underwent a complete check by the professional company that did the porting job. So I guess I can exclude a crack in the head. The inlet manifold was new, should be Ok also. I checked the butterflies also and these looked perfect. The butterfly axles have been bushed during the refurbishment of the carbs which was done by a well known professional also.
I agree with you, as the idle is rising with the temperature of the engine it’s most likely that there must be an air leak somewhere. Worst case would be a crack in a carb‘s housing but to me it is hard to imagine that the professional who did the refurbishment could not have recognized that. I still have one point to check which is the brake servo and it’s connections.

Chris

[MMT1968]

I just have installed a second spring and adjusted the throttle linkage again. There was an absolutely minimal play of maybe half a millimeter in the linkage of the rear carb. On a short ride for testing it seems to be better. The idle is not rising but the second spring makes the throttle reaction a bit harsh. I keep working on it.

Chris

[Guildbass]

Thinking about this, I've had a similar thing happen. Both my Rover 2000TC and the VX490 we built for college had twin SU's and that 'hanging' revs thing was caused by one carb's idle speed setting being slightly higher than the other. The carbs both opened seemingly simultaneously and the slides lifted together, but because one idle set was too high, that carb held the engine revs up when they shut
Your mention of the extra spring now makes sense as the linkage is pulling the 'high' one down harder and reducing the difference.
I bet if you put a screwdriver onto each, one would feel tighter and that one is going to be the 'high' one. I would imagine any air leak from outside the manifold is going to effect both, and unless the mixtures are rich, is unlikely to cause an increase and even less likely in just one.
I would try a half or quarter turn on each idle speed screw in turn, snapping the throttle each time. Until it drops to idle smartly. If the idle threatens to die, it's probably the other screw that's high!
Failing that, grab a cheap column manometer and run two tubes off each of the carb vacuum ports. It's a very easy way to confirm both are pulling the same at idle, and adjustment is as simple as backing off the errant set screw until the readings match .
I enjoy setting carbs, most of my bikes were twin carb, and my Alfa had two twin chokes but each choke had its own mixture so essentially four carbs. A four column manometer made setting them a cinch!

[philthehill]

A piece of rubber pipe held at one end against the carb intake and the other end against your ear will by the sound of the air going into the carbs tell you if the carbs are synchronised.
May be cheap and cheerful but is very effective.

[MMT1968]

Thanks for that good post.
As I wrote earlier the idle setting screws are fully turned out and I can increase idle speed only by turning them.
But. What Ihave found is that the butterfly axle connecting both butterflies ist turned to a complete stop by just one spring. When I try then to move the butterflies of each carb the one of the rear carb can be closed a tiny bit more by a screwdriver pressing on it. That is only 1 or 2 tens of a millimeter and I have to press really hard to move it. At the front carb I can’t move the butterly any more. I guess I‘ll dismantle that carb and centralize the butterfly again. I had checked it with a torch and that very small rim of light was regulary all around and equal in both carbs. But maybe there is some resistance that prevents the butterly from closing completely when just under the load of one spring.

@Phil,
I have done this already and the carbs seem to be synchronized well.

Chris

[philthehill]

If the carb butterfly's are centralised correctly there should be no possibility of light been seen around the rim of the butterfly.

[MMT1968]

I‘ve checked this again and it looks as you write. No light through the closed butterfly.
Then I adjusted the linkage of both butterflies once again but this time beginning with the rear carb and after that the front one. Now it’s idling at some 1000 revs while warming up. When the engine is warm ( oil and water) and I let it run for a few seconds at constant revs say 2000 it doesn’t fall to idle when I step off the accelerator. Then I can give it a push with a screwdriver or a short step on the accelerator and it falls to idle again. Seems as if there is some resistance that keeps the butterflies from closing completely if the engine is warm. The movement of the butterfly axle is just 1 to 2 tenths off a millimeter then.
Could it be that the bushes of the rear carb’s butterfly axle are a bit too tight and inhibit the axle moving more when warm?

Chris