Open modified questions

K

KartRacer89

Member
Alright guys, I wrapped up an open modified this week and ran it for the first time last night. I had some technical difficulties that prevented me from finishing a race, but does show potential in being able to keep up with the OHV clones.

With that being said, one of the main things is that it seems to idle high, I’m talking 3500-4200 rpm range. I turned the adjuster on top all the way down and the idle speed adjuster is turned out pretty far, the slide opening at idle is about 1/8 inch or little more. Being that this is my first time having an open modified flathead, I am clueless as to what the idle speed should be with the Mikuni carb. I don’t know what size pilot is in it at the moment, I have yet to open it up and look at the jet sizes, I have to open it up to clean it out anyway and will update later with the jet size.

The other question I have is clutch engagement, I had to borrow a 3 disc 6 spring Bully from my brother last night and after a lot of adjusting it seemed to engage between 4500 and 5000. I have a 3 disc 4 spring Bully that’s getting rebuilt at the moment and I told the builder what engine it’s for, but I know I’ll have to set it once it’s on the engine.
 

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I would guesstimate that the idle may be from too short of throttle cable causing it to idle high or you have a air leak somewhere.
 
Ok so I’ve been told that my motor isn’t an open modified. The bore is 60 over from standard and using standard size valves. The eyebrows are shaved so it’s not a limited but a modified.

With that out of the way, I’m still looking for advice and tuning help. Thank you to members that have responded.
 
So you have slack in the throttle cable when the slide is shut? Try taking the the cable off the slide to see if the slide closes even farther with the slides idle screw out or off of the carb. If the air door is shut the monster can not breath and will die.
 
Freeze, yes there is a small amount of slack in the cable, the slide is completely closed and the engine still wants to idle between 3500 and 4200 at start up.

At what rpm should the clutch be engaging? I’m guessing 4500 to 5000?
 
KartRacer89 said:
The slide is all the way shut with no throttle applied, checked it multiple times yesterday and today. The largest gap in the middle of the throat is about 1/8 inch when fully shut.
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1/8th inch is too much . I would think it should be closed off completly .
Any possibility the slide been modified ?
 
Nothing wrong with idling around 3500RPM. Clutch engagement around 4500 is fine also. Try spraying WD-40 around the outside of the carb and intake while it’s idling. If the RPM’s drop you have an air leak somewhere. As for jets, what temps are you running? You can properly tune a methanol engine unless you know the temperature it’s operating at. You can use head temp or exhaust gas temp. I prefer head temp because that’s what I’m used to. EGT is probably more accurate.
 
Flattop, the slide in your picture looks like it has a larger opening than mine. The pic I put is the slide all the way shut and the idle adjuster on the side is not raising the slide, it is backed out. Maybe the slide was changed out at some time but I don’t know because I didn’t buy it new.

I’ll probably put a bigger idle jet in, me personally I would like it to idle lower.

Ski, I didn’t make enough laps to know how hot it’s running so I don’t have that piece of data. I know for stockers, 450 and up is the high end and ideal temp range is 300 to 400.
 

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Yes it should definitely idle lower. 2000-2500 should be easy enough with the engine set-up .
There's a air bleed screw in the throat . It could have a effect . Put the biggest pilot je you have in , test .
 
flattop1 said:
Did you find a solution for the idle problem ?
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I have some larger pilot jets coming. Haven’t raced or run the engine since then. Hoping a larger pilot will drop the idle rpm to what I want it to be. Just got my clutch back from our local clutch guy and he set the stall at 4750. Might be racing this thing again in a couple weeks.
 
I love M5. It seems to work really well in both my flatties and animals. I used to run M1 1 gallon to 1oz of nitro. I think it pulls better on M5 than the M1/nitro blend.
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