RV8 tuning

[oldcobber]

Hello all,

Can anyone help? Just finished a winter re build, started first time and ran ok at 2000 rpm for 15 minutes to run the cam in. Had to stop for a minor top hose problem. Ran for another 5 minutes at same rpm and everything looked good. Had to leave it then for 2 weeks due to lack of time.

Started the car at the weekend to check timing and mixture etc but could not get the tick over down below 1500 rpm.

I have tried timing, mixture, checked breather hoses, vacum etc all to no avail. Engine is 3.5, lightened flywheel, fast road cam, mallory electronic and edlebrock/weber carb and manifold. Idle screw is in fresh air.

Thanks

Bob

[wilf]

Is the throttle being held off the idle stop by a too-short cable?

Is the fast idle cam jammed in position?

Do you have electric choke - if so is it coming "off"? (I would junk it anyway).

Luck sorting it out.

[jrv8]

Hi,
Check that you have the vacuum advance connected to the "timed vac port" on the carb and not to the "full vac port".
If you get these the wrong way round the engine will getl full ignition vacuum advance, as soon as it fires up.
Also check for air leaks, and make sure that all spare ports on the carb are blocked off.
Regards
Jim

[oldcobber]

Hi guy`s,

Thanks for the tips.

Throttle cable, cams, levers, choke all ok.

I think i`ve got the vacum on the right out let, but its about the only thing i`ve not tried so will swop ports anyway.

Thanks

Bob

[Sidecarbod]

Originally Posted by oldcobber

edlebrock/weber carb and manifold. Idle screw is in fresh air.

Thanks

Bob

Hi Bob,

Do you mean that it won't run, i.e. stalls below 1500 rpm or that you just can't get the revs any lower than 1500 rpm? I think Wilf and Jim think the latter but incase its the former you could check the following:-


Not sure what you mean by "fresh air" but if the screws aren't set right or at least nearly right its not going to run too well when it tries to pull fuel from the pilot circuit.

From memory (I'll check when I get home) the screws need to be round 2 turns out but this could go up to 3+3/4 turns, it depends on your cam and other things.


I set mine by having then quite a long way out then gradually winding the in whilst listening to the revs. The revs might go up slightly but at some point will start to fall as the motor goes lean. I good start is 1/2 a turn back out from where the lean drop occurs.


8 BTDC is a good start for the ignition but might have to be 6 if you run soft advance weight springs. (your dizzy is different to mine). I found that whilst setting up the ignition it made no difference whether the vac pipe was connected or not. It certainly makes a difference just off tickover. (You could pull it off and plug the end to be sure whilst checking with the timing with a strobe).

Regards,

Pete

[wilf]

Pete - I took it to mean the idle stop screw being in fresh air, not the idle mixture screws (if they were right out I don't think it would run at all). That is why I started asking about anything which could be holding the carb butterflies open.

Now I suspect a vacuum leak.

[Sidecarbod]

Originally Posted by wilf

Pete - I took it to mean the idle stop screw being in fresh air, not the idle mixture screws (if they were right out I don't think it would run at all). That is why I started asking about anything which could be holding the carb butterflies open.

Now I suspect a vacuum leak.

Hi Wilf,

I think you are right, I had it in my head that Bob was refering to the mixture screws. If the idle stop screw is floating about in fresh air then something must be holding the carb open or the carb is fully closed and the screw is backed right off. Then there must be a leak to allow the motor to run. (This would expalin how the motor gets air but I'm not sure how it would get fuel).

It would be worth removing the carb from the manifold to check that it closes properly.

Regards,

Pete

[oldcobber]

Hi guys,

It starts no problem and runs at about 1800 rpm straight away. The idle screw is off the carb cam and i cant pull back (rotate) the carb cam any further in its normal position.
The mixture screws I have screwed in (weaker I think) as far as I can before the engine starts to run rough.
If I try altering the timing it just runs rougher and tries to keep the same speed.

Had a horrible thought.......If the follower pre load was really bad (I had to use two shims under each post, and I got it wrong), what would the result be?

Bob

[TINKA]

Just because you have removed the Idle screw and it is sat on the cast frame of the carb does not mean the butterfly is closed these things have been known to get bent. Make sure the butterfly is actually full closed and bend ( if necessary ) the linkage accordingly.

[Sidecarbod]

Originally Posted by oldcobber

Hi guys,

It starts no problem and runs at about 1800 rpm straight away. The idle screw is off the carb cam and i cant pull back (rotate) the carb cam any further in its normal position.
The mixture screws I have screwed in (weaker I think) as far as I can before the engine starts to run rough.
If I try altering the timing it just runs rougher and tries to keep the same speed.

Had a horrible thought.......If the follower pre load was really bad (I had to use two shims under each post, and I got it wrong), what would the result be?


Bob

There is no way your pre-load is going to cause the motor to run at too high an idle speed.

I still think that you should pull the carb off and chack that the butterflies are fully closing, They should only be open a nants fart when the motor is ticking over. IMHO even if the timing was causing the engine to run at too high a speed the carb should be always be able to close to the point where the engine simply can not get any air or fuel to run...it would just have to stall.

Just had a thought...maybe the secondaries are sticking open?

Where abouts do you live? if its near me we can stick my carb on and then be sure one way or the other.

Regards,

Pete