Carb Sync above 3500 RPM?

by Ron Lalonde in General Tech Discussion 8 years ago

Hi Everyone;

Have been synchronizing my carbs with a dual manifold pressure gauge. As per Roger Lees advice, I sync at 3500RPM, and my idle speed is set between 1800/1900 rpm.
Last night, after completing the sync, I brought the RPM up to 4000 and my sync was off a tiny bit. (two needle width).
Have never done, nor tried, to sync above 3500 RPM. Have always thought that if it was bang on from 3500 and below that the higher RPMs would be fine as well.
Is this normal? Should I be concerned?

Anyway, I would also like to thank everyone on this form for being such an invaluable learning tool for maintaining my engine.
Special thanks to Roger Lee and Rob Seaton who always go above and beyond to help everyone!!

Thanks again

Ron Lalonde
601HDS/912UL
Debert, Nova Scotia
Canada

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Re: Carb Sync above 3500 RPM?

by Rob Seaton » 8 years ago

Rob Seaton

3 1.32k 240

Roger and I (that is to say "the rest of the known universe") disagree about what RPM to sync at. ;)
Roger has good reasons for his technique, I will let him defend them.

For the rest of us: the whole idea behind syncing the carbs is to have a smooth idle (where the piston pulses are the strongest, resulting in the gearbox working very hard and the whole engine is subject to vibration) So you should set the sync at 2000-2000rpm and check the sync at idle. If the carbs are vibrating too much to even get the sync close then you can increase the rpm to bring the gage needles together. It will be very windy! (tractor type configuration). But then go back to approx 2000 to finish the job.

Anyways, to answer your question: If you have a perfect sync at idle but see a difference in the needles at higher rpm this is unusual and may indicate a carb issue such as a sticky slide or contamination, an intake manifold leak, valve issue or compression issue.

Thank you said by: RotaxOwner Admin

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Re: Carb Sync above 3500 RPM?

by Ron Lalonde » 8 years ago

Ron Lalonde

4 9 0

Hi Rob;
Thanks for your quick answer. I will check for the conditions that you mention. The engine seems to be running so damn good/smooth (that I hate to fix it till its broken)

As far as windy, I do my carb sync from inside my airplane. My dual manifold pressure gauge stays in the cockpit, the hoses run through the firewall to the connections.
Shut off the engine to "tweak" the cables.

I might even try getting the readings with the cowling on, was kinda hoping the additional (high rpm) airflow over the carbs (vent lines etc) might be an issue. Will checkout and clean the carbs, and compression check after that. Simple checks first.

Thanks again

Ron

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Re: Carb Sync above 3500 RPM?

by Rob Seaton » 8 years ago

Rob Seaton

3 1.32k 240

Sound good Ron,
still waiting for Roger to strike back.... the silence is killing me.

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Re: Carb Sync above 3500 RPM?

by Roger Lee » 8 years ago

Roger Lee

125 5.12k 4.53k

Hi Rob,
LOL
I love our conversations. ;) :cheer:
I learn something all the time and look forward to them.

Just so everyone knows I'm a huge believer in carb syncs and once you understand what the gauges are telling you they are fairly quick and easy.
I prefer to sync at the plane's idle setting and then between 3300-3500. The reason I pick the higher rpms over 2000-2500 is that if I use 2500 and then advance the throttle the vacuum gauge needles almost always split. If I sync up around 3300-3500 rpm then they tend to stay together if you advance the throttle and watch the needles. So where do you run an engine usually? At idle and then in the higher rpms and not back down in the idle circuit around 2000-2500.
I do not ever sync above 3500 because of the crossover tube size.

The one reason I think the needles start to split again after 2500 is at those lower rpms it doesn't flow enough air/fuel to be representative of the higher rpms. You're in the idle circuitry and not starting to engage the main jet/needle until you get up into the 3000's. So air and fuel just don't represent the engine at higher rpms.

So for me setting the sync for idle and 3300-3500 is where I go.

This all said it may boil down to personal experience and personal preference.

Just so you do syncs that would be good. If you are in the 2500 rpm camp that's good. If you're in the 3300-3500 camp that's good too.

Just get'er done. :)

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: Ron Lalonde, RotaxOwner Admin

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Re: Carb Sync above 3500 RPM?

by Chris Culler » 3 years ago

Chris Culler

3 16 41

"I do not ever sync above 3500 because of the crossover tube size."

I wish to understand that statement. As I understand it, the crossover tube is least effective at high RPM, and during sync you have it blocked entirely. How would c/o tube size have any relevance to the sync process?

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Carb Sync above 3500 RPM?

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