Topic: Carb Help After Complete Rebuild. I Will Get These Things Dialed Come Hell.....

[MattFreeman]

First off:
'76 F1
836
"hot street" cam
oversize intake valves
Stage 1+ MR port
factory 4 into 1 w/mostly open muffler.
105 main (stock)
42.5 pilot (+1 size from stock)
Needle clip in the middle.
Air/Fuel screws out 2 turns
K&N (gasp) pods.
Pamco Ultimate Ignition.

Bike (tank/seat) not finished so can't be ridden just run on a stand.

Here's the story.
Try to start bike for first time, wont idle.
Turn idle screw out from factory. From 1.5 turns to 2 turns out.
Starts easy, idles at 1500 to 2000. Stall under 1300 or so when using the linkage screw to adjust idle speed.

I get the timing close but cant get it perfect because it won't idle under 1000.
Spend aprox half to 1 hr using homemade manometer to get carbs balanced in a 60 degree garage.

Get vacuum balanced.
Bike gets really hot, oil pressure drops. Probably ran hot for awhile before I noticed.

My research got me thinking that I was running lean but I pulled the plugs and they are lightly carbon covered, suggesting rich? This is, of course, after having the carbs/rpm's all over the place trying to use my homemade mano.

So what I think I want to know is:
A. Should I be able to get this built motor to idle under 1K? If so what are some ways that
I could do that?
B.  Is there a way to find optimum air/fuel screw placement?
C. I could also use some suggestions on cleaning the plugs so I can reuse them. I think I'll be through this process more than once.
D. Other suggestions, or words of encouragement?

I would like to get the idle down first so I can dial timing then run and check it again with new plugs.
I am also hoping that the bike overheating/dropping pressure is a carb issue so any advice regarding that idea would be appreciated.

I appreciate any input. However, I fully intend to get this old girl running with the pods, so.....

[harisuluv]

So, you know the bike is air-cooled right?  It will overheat if it's just sitting there not moving. 

I'm not well versed on setting an electronic ignition so I will let someone else chime in on that.

However, your carbs look to be essentially stock with the exception of the slow jets.  Your bike is heavily modified.  Your carb settings are probably no where near adjusted, if you had only changed to pod filters from stock and nothing else you would still be way off.  You at least need a needle clip change, to 4th from the top if not 5.  Then probably somewhere around a 130 main jet.  I'm not sure about the pilot jet.  You might be ok where you are with that 42.5 for now.

What is a "mostly" open muffler?  Is a part of it chopped off?  Or do you have just the 4-1 headers?

[Lostboy Steve]

Just put a decent size fan in front of it. Thats what I do. Then you can screw around and not worry.

[Lostboy Steve]

My stock engine doesn't like to idle under 1000rpms. It never has. Anything under 1000 and it feels like the starter is trying to engage. I leave it at just about 1000rpms. Depending on the season I have to fine tune it with the idle screw as the temperature changes. It seems to be a very cold blooded engine. Until its completely up to operating temp it doesnt like to idle low, period. Thats my experience anyway.

[MattFreeman]

I know that overheating the engine was my fault as far as letting it idle for so long, I got really wrapped up in using the mano which was a pretty finicky piece of equipment. I've already got a fan out for the next time.

I expect to have to do jetting changes once I am able to ride the bike.

The mostly open muffler is the cycleX race slip-on. It is "slightly baffled" (like me) but is essentially open.

Putting the whole convoluted story out there may kind of distracted from what I would like to precede with...

The factory air/fuel screw setting obviously won't work so is there a way to find it's optimum setting?

And

Should I be able to get this bike to idle at its factory recommended 900ish rpm? I can understand if not but if it should be possible I would like to try. So far about 1200/1300 is the best I can do.

Thanks for the input thus far.

[MRieck]

Don't be obsessed with getting it to idle under 1,000RPM Matt.....it will not happen with your set up. 1,200 to 1,300 is fine. Mine idles at about 1,600RPM.
 That mechanical tach isn't that accurate either. Make sure you have no vacuum leaks and you can screw the mixture screws in to 1/8 of a turn.
 This is where an A/F meter really helps.
PS...I don't get the obsession with a low idle....it kills oil pressure which is not good for a plain bearing crank.

[Don R]

Your motor may calm down a little after it runs in, A fresh rebuild takes some time for all the parts to get to know each other.

[MattFreeman]

Thanks guys! That makes me feel better.

Mike:  Are you saying that I should turn my a/f screw in 1/8th of a turn to 1 7/8ths out from seated? and what is your opinion on the size main jet and needle setting I should run?

Thought a low idle might make it sound like a Chevy small block with a hot cam.

[MRieck]

Thanks guys! That makes me feel better.

Mike:  Are you saying that I should turn my a/f screw in 1/8th of a turn to 1 7/8ths out from seated? and what is your opinion on the size main jet and needle setting I should run?

Thought a low idle might make it sound like a Chevy small block with a hot cam.

I would change the main to 110.Leave needele the same . Screw the pliot/idle screw all the way in....GENTLY....and turn it out 1 turn. Start there. You'll be fine.

[Davidov]

Don't be obsessed with getting it to idle under 1,000RPM Matt.....it will not happen with your set up. 1,200 to 1,300 is fine. Mine idles at about 1,600RPM.
 That mechanical tach isn't that accurate either. Make sure you have no vacuum leaks and you can screw the mixture screws in to 1/8 of a turn.
 This is where an A/F meter really helps.
PS...I don't get the obsession with a low idle....it kills oil pressure which is not good for a plain bearing crank.

I checked the factory tach against a digital pickup style tach, and 1200RPM on the Honda tach was about 800 (actual)RPM.

I'm giving my 78 750F a full break in period of a few hundred miles before I sweat the idle speed. 

[lrutt]

I gotta say, even a 110 main sounds WAY WAY lean for this setup. I'd be thinking something north of 125 to be honest, just to start. good call on the pilots though.

[Old Scrambler]

Just from my experience with my brother-in-law's bike.............700-ft altitude............836 kit with cam in a K4..........10.5 to 1 compression............pods, header, .........started fine but would not idle with 120 mains in stock carbs.............went down to 117.5 mains and had detonation above 5000 rpm............went up to 125s and 42.5 pilots..............ran fine but the piston damage was apparent.............#2 & 3 lost the edges at the exhaust ports............solved the problem by going back to stock......Completely stock............and sold the bike to a buyer in Australia. 

My advice..........+1 with Irutt..............start on the rich side of risk and tune DOWN to protect the motor.  If you live somewhere between Denver and Pikes Peak............the 110s may be good.

[martin99]

Don't be obsessed with getting it to idle under 1,000RPM Matt.....it will not happen with your set up. 1,200 to 1,300 is fine. Mine idles at about 1,600RPM.
 That mechanical tach isn't that accurate either. Make sure you have no vacuum leaks and you can screw the mixture screws in to 1/8 of a turn.
 This is where an A/F meter really helps.
PS...I don't get the obsession with a low idle....it kills oil pressure which is not good for a plain bearing crank.

I checked the factory tach against a digital pickup style tach, and 1200RPM on the Honda tach was about 800 (actual)RPM.

I'm giving my 78 750F a full break in period of a few hundred miles before I sweat the idle speed.

A difference of 400rpm? Wow. Is that typical? I didn't expect the factory tachos to be very accurate, but that seems way out. Anyone else able to check theirs for comparison? Does the difference increase relative to engine speed? I'll bet most check full advance at 2500 (or whatever the manual calls for) but how many realise they might actually be checking against, say, 2100? Pretty important if you're trying to get a decent state of tune. I don't have a digital reader myself, otherwise I'd be out there right now

[MattFreeman]

So far I'm not surprised that the analog tachs are not perfect.

I feel good about idling at 1200/1300.

Detonation scares me. I've already overheated this thing...

It seems intuitive that with all this air I would want more fuel, but I have it with the bigger intake valves and I think with the cam too....?

I'm tempted to change the title this thread to "this idiots guide to tuning carbs in a hot motor".

[harisuluv]

I already gave you all the settings for a good starting point. 

[MRieck]

I just reread the original post and saw you have K&N's (originally I thought it was the stock box). In that case the mains need to be much larger than 110. I agree with the 125 to 130 range. I apologize for the error.

[MattFreeman]

Thanks Mike!

I was pretty conflicted there for a minute.

[MRieck]

Thanks Mike!

I was pretty conflicted there for a minute.

Sorry about that Matt....I blew through your original post to quickly. In the instructions supplied with K&N individual filters recommends going going up 15% on the main with a stock exhaust ie 110 to a 125 main. My FJ went up 20% (112 to 132) with a Stage 3 Dynojet kit, Yosh Stage 1 cams and an effective pipe.

[MattFreeman]

Up and running!!! Runs, shifts, stops. 4.5 years in the making.

42.5 idles, 130 mains. Screws 2 turns out. Needle clips in the 3rd spot.

Getting a hesitation when going from closed to open. Is that an issue that will get fixed by needle adjustment?