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Topic-icon Carburetor balancing

  • Mufflerbearing
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3 weeks 4 days ago #5585 by Mufflerbearing
Carburetor balancing was created by Mufflerbearing
Hi gang,
I’m a first time poster to this forum. I recently won an 1980 R65 in Raleigh NC and am trying to get the bike tuned properly. The bike has about 45K, looks to be in excellent condition. Compression is good at 126 psi in both cylinders. New plugs. It starts reliably with choke/no throttle needed. I’ve set the points and adjusted the valves. The bike initially would hang up at higher rpm due to the mech advance hanging up I believe so while doing the points I removed and lubed the advance unit. Now rpms come down properly when off the throttle.
It just always felt like I wasn’t getting even power delivery from both cylinders. Additionally, acceleration flattens out around 4000rpm or so. If I point a heat gun at the exhaust nuts, the right side is always about 50 degrees hotter than the left.
The mixtures were both near 11/2 turns out.

In an effort to get her running better. I rebuilt both carbs with all new rubber O rings and gaskets (Thanks Bob’s!). Everything looked clean inside. Only thing suspect was floats were a little off(more fuel in bowl) which I corrected. Did not find any clogged jets.
So.....in an effort to more accurately adjust carbs I attached a carbtune to look at relative vacuum. The Right side is pulling WAY more vacuum but I don’t why. Can’t find any vacuum leaks using the propane method around all connections.
Any thoughts ?
I can tell this motor can run great, it’s just not adjusted right as yet.
Thanks for any input fellow airheads.

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3 weeks 2 days ago - 3 weeks 2 days ago #5588 by Wobbly
Replied by Wobbly on topic Carburetor balancing
Welcome Aboard !

You'll want to...
• Run fresh high test namebrand fuel
• Strobe time the ignition at high RPM
• Check the diaphragms in the carbs for holes or rips
• Update to the latest carb floats and float needles
• Build one of these..... www.powerchutes.com/manometer.asp
...and balance the carbs at 1000 RPM with the Idle Screws, and again at 1800 RPM using the cable adjusters

Then Look Right Here to see if there's anything else you need.

Hope this helps.

Don't hide 'em, Ride 'em !!
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Last edit: 3 weeks 2 days ago by Wobbly.

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3 weeks 2 days ago #5589 by Mufflerbearing
Replied by Mufflerbearing on topic Carburetor balancing
Thanks for the response Wobbly. I did attach a Carbtune (Manometer) again today and discovered I needed to do quite a bit more tweaking of the throttle screws to get the vacuum balanced, but once I did she is much smoother and I no longer have the flat spot at around 4000 rpm. Mixtures are a little rich for now (about 13/4 to 2 turns). I forgot to mention in first post that I measured primary and secondary resistance of the original coils and they were both a little below spec in the Clymer manual. I have an aftermarket Dyna coil I plan to install next and see how that affects the performance as well. Thanks again for the reply!

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3 weeks 2 days ago - 3 weeks 2 days ago #5591 by Wobbly
Replied by Wobbly on topic Carburetor balancing

Mufflerbearing wrote: Thanks for the response Wobbly. I did attach a Carbtune (manometer) again today and discovered I needed to do quite a bit more tweaking of the throttle screws to get the vacuum balanced, but once I did she is much smoother and I no longer have the flat spot at around 4000 rpm. Mixtures are a little rich for now (about 1-3/4 to 2 turns).

Thoughts...
• By "throttle screws" I assume you mean the Idle Speed Screws.
• Adjusting the Idle Speed Screws will have zero effect on 4000 RPM. That's what the cable adjustment is about. You made no mention of that. Was that done ?
• Sorry, but the Idle Mixture Screws have almost zero effect on running "a little rich for now". And how was that determined, anyway ?

Mufflerbearing wrote: I forgot to mention in first post that I measured primary and secondary resistance of the original coils and they were both a little below spec in the Clymer manual. I have an aftermarket Dyna coil I plan to install next and see how that affects the performance as well. Thanks again for the reply!

Thoughts...
• I was always taught that if it ain't broke, then don't fix it.
• So you believe a 3rd party manual in place of an OEM part ? You are free to believe what you want, I'm simply interested in the thought process that got you there. Why would you want to make dramatic changes to an engine that seems to be responding to standard tuning techniques ?
• The measurable effects of a Dyna coil are going to be negligible. Metrics are only going to be gathered in 1/4 mile times and overall fuel mileage. The R65 is not a 1/4 mile bike. And since the carbs are still in a state of being washed out after long non-use, fuel mileage is going to be radically changing even if you do nothing. So I'm curious as to this "plan".
• And the plan is to swap the coil without replacing the 40 year old wires and plug caps ? The wires and plug caps are known problem areas due to aging. I'm again just curious.
• I note that setting the ignition timing was in the list of suggestions. No mention of that was made in the reply, and yet it's probably the one thing that holds the most control over engine smoothness, throttle response, and fuel mileage. Just sort of seems like factor #1 is being ignored in favor of factor #87. Again, it's your machine, I'm only interested in the thought process.

Glad it's running better. All the best.

Don't hide 'em, Ride 'em !!
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Last edit: 3 weeks 2 days ago by Wobbly.

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2 weeks 6 days ago #5603 by Mufflerbearing
Replied by Mufflerbearing on topic Carburetor balancing
Geez Wobbly, easy does it!
Yes, I meant idle screws not throttle screws.
Regarding running rich, I can actually smell a change after enrichening the mixture, plus I get a few pops from the exhaust when closing the throttle from higher RPM from unburned fuel.
I did switch to an aftermarket Dyna coil which seemed to help it run a little smoother.
Regarding the timing, I did say I checked it in my original post. I spent a LOT of time getting this right including checking the static and dynamic timing.

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1 week 5 days ago #5616 by Cubdriver
Replied by Cubdriver on topic Carburetor balancing
Contrary to my own intuition, I’ve been led to believe that popping on deceleration with a closed throttle was likely due to the IDLE mixture being set a bit LEAN. Of course this would have no effect above idle speed. True?

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