lean mixture problem

Ian Z · Post by **Ian Z** » Fri Mar 16, 07 7:20 pm

Before I trawl through the libraries, anyone got any pointers for a lean mixture problem I have?

Since rebuilding the 750vs (thanks Neil Wil) I finally sorted a poor idle problem out but cant get rid of a lean condition off idle between 2k-3k revs. Below and above this range its ok. It breaks up noticably under load at this range = a beggar to drive round town- and obviously only slightly noticable at standstill. Kicks in good from 3k upwards.

The reading is between about 0.06-0.2 volts air/fuel. Beyond 3k its fairly stable peaking around 0.9 at standstill. Need to check under load also.

Any pointers? cheers

Kev · Post by **Kev** » Fri Mar 16, 07 8:16 pm

bigger shooter. Is the cam underneath to the shooter pump adjusted correctly. At idle if you touch the cam and give it a little push do the revs drop?

Dave-R · Post by **Dave-R** » Fri Mar 16, 07 8:17 pm

Double pumper.

Ian Z · Post by **Ian Z** » Fri Mar 16, 07 10:27 pm

I will check the squirt, but the break up is constant at steady throttle, not just when it opens up. A new carb is too easy, Dave! (and off my budget for this year). Any particular fuel/air circuits worth double checking? It was ok before I stripped the carb. I do know the vacuum at the baseplate is about 5-6inhg (4.5 pvalve), but need to check this through the rev range. Any iseas what the vacuum should be from experience?

Anonymous · Post by **Anonymous** » Sat Mar 17, 07 2:17 am

Speak to Neil

Check yer transition slots are set right, around 20 thou, as you may well be runnin on main metering, causing the motor to overfuel when you stomp on it.

I take it you reinstalled the NRVs on the shooters.

If all was well before you stripped the carb, then all should be Ok on the rebuild.

Another cause could be the pump squirter to big, causing the engine to bog due to less duration time of the squirters.

If the vaccum @ idle is as low as 5, then you need a 2.5/3 power valve, any higher than this, then you'll get a stumble as the power valve will open to soon.

Dave-R · Post by **Dave-R** » Sat Mar 17, 07 10:36 am

Ian.
As Adam says, you should check that the throttle blades are covering the transition slots at idle.

These slots sooth the transition from the idle circuits to the main jets. That could well be your problem.

Adam is also correct on the power valve rating. You need a rating of at least 1.5 to 2 inches below your manifold vacuum.

If both of these things are OK then there is probably something wrong but I can't think what. Maybe a wrong gasket or something?

Ian Z · Post by **Ian Z** » Sat Mar 17, 07 3:43 pm

To get an idle at 1k-1.8k, the curb screw was holding the primaries open. So I opened the stop on the secondaries a fraction to let light thru on the secondaries, then it would idle cleanly at 1k with the primaries fully closed. Gives a good mixture at idle now. Off idle at standstill it gets progressively leaner up to about 2k then evens out at 3k. Looks similar under load as well, so it looks like the primary metering is bad in this range. 3k up its spot on.
Appears to be squirting fine, and has got return valve ok and checked pump lever ok - 0.015?. Got a better vac gauge now and it shows 7inhg at idle, so 4.5 pv not too bad.
Will strip and blow thru when it stops bleeding raining! It has to be blocked somewhere me reckons.
I'm still re-learning after all the years doing little with cars...

Dont wanna spend on decent carb just yet if i can help it and until I can get no further!!

Anonymous · Post by **Anonymous** » Sat Mar 17, 07 4:52 pm

I don't think you set the power valve rating based on the manifold reading at idle.

If I remember correctly, you use the cruise vacuum reading.

Dave-R · Post by **Dave-R** » Sat Mar 17, 07 5:29 pm

Why do you want to idle as high as that? 850-900 should be plenty enough.

Anonymous · Post by **Anonymous** » Sat Mar 17, 07 5:37 pm

You might wanna blow through the PVCR, as these will cause a lean out @ part throttle if there blocked.

You could remove the carb, flip it over & check the primary transition slot is 20 to 30 thou open, then check the secondary, which should be around 20 thou.

Whats the initial timing figure? as the initial increases, you can shut the blades down even further.

Check the vac @ around 900 rpm, in neutral with no load, then select a P/V rated @ 2 below vac reading for auto & 2.5 below for manual.

You can always play with the P/V by shimming it, or cutting the spring down

Good luck

Ian Z · Post by **Ian Z** » Sat Mar 17, 07 6:41 pm

thanks guys, will try again with the advice.

Dave-R · Post by **Dave-R** » Sat Mar 17, 07 7:36 pm

Adam68 wrote:You could remove the carb, flip it over & check the primary transition slot is 20 to 30 thou open,

I think that is a max opening figure. I am pretty sure it can be as much as fully closed going the other way.

Anonymous · Post by **Anonymous** » Sat Mar 17, 07 7:54 pm

40 thou would be as close as you wanna go, flipping the carb over was to check what they were when ideling @ 1000 revs

Bringing the initial up will enable the the curb idle to be shut down, all depending what it's set @ the mo.

Ian Z · Post by **Ian Z** » Sun Mar 18, 07 12:05 pm

Done, thanks. Got the butterflies set and blew thru metering block and other passages. Think it was an obstruction in the metering block. Idle screws now having a marked effect! 7inhg with 4.5 pv, fuel pressure 6min 7max psi.. Mixture ok now from idle upwards, just need to test under load for fine tuning and check secondaries opening. Godamn hail stoning now

Dave-R · Post by **Dave-R** » Sun Mar 18, 07 12:25 pm

Well done!