The V8 Owners Forum -

I've recently been running my Rush on the road now I'm through SVA and have noticed some odd idling / running characteristics as follows:

The motor is a 3.5 RV8 with Edelbrock 500 and dual plane Edelbrock manifold.

1. When the car is fully warmed up and I go out of gear, sometimes the engine idles at approx 1,500 rpm and sometimes at about 800 rpm (fine) and sometimes fades away and the engine cuts out unless I notice it happening and quickly tap the throttle. It appears purely random which 'mode' it chooses to adopt. I have done a rod and jet change from stock, but cannot remember what I fitted - will have to go home and look this up.

2. I also notice when driving at about 1,500 rpm the car bogging slightly (particularly in 4th & 5th) on gentle throttle action, a bit harder with the pedal gets through this. I assume this is a rod/jet lean vs rich problem after reading the recent threads on rods-n-jets-n-stuff.

So...How best to work through these ?

Alex

Find out what rods and jets you have and I can tell you how much richer/leaner than stock you are running.

It could be some dirt in the idle jets. With the engine off, screw the idle jets in all the way and count how many turns they go in. Then you can just unscrew them and wipe off any dirt. Then put them back and adjust to where they originally were.

Can you hear a whistling sound when the idle jumps around?

v8alex wrote:I've recently been running my Rush on the road now I'm through SVA and have noticed some odd idling / running characteristics as follows:

The motor is a 3.5 RV8 with Edelbrock 500 and dual plane Edelbrock manifold.

1. When the car is fully warmed up and I go out of gear, sometimes the engine idles at approx 1,500 rpm and sometimes at about 800 rpm (fine) and sometimes fades away and the engine cuts out unless I notice it happening and quickly tap the throttle. It appears purely random which 'mode' it chooses to adopt. I have done a rod and jet change from stock, but cannot remember what I fitted - will have to go home and look this up.

2. I also notice when driving at about 1,500 rpm the car bogging slightly (particularly in 4th & 5th) on gentle throttle action, a bit harder with the pedal gets through this. I assume this is a rod/jet lean vs rich problem after reading the recent threads on rods-n-jets-n-stuff.

So...How best to work through these ?

Alex

Like Richard said, we really need to know what jets and rods are fitted before anyone can help.

Everything that I know about these carbs (I've worked on quite a few now!) is here:- (Under the fuel pump gumph)

http://how-to-build-a-pilgrim-sumo.wiki ... ing-system


Whilst your at it have a read of this assuming that you are running a dizzy:- (under the gumph on braided hoses)

http://how-to-build-a-pilgrim-sumo.wiki ... by-members.

The reason that I suggested a read of the second link is that at the end of the day only two things make a correctly built engine run, fuel (plus air) and sparks. Get them both right and your lump will give its best!

Pete

The idle jets appear clean (the carb is brand new out of box), but I have checked anyway. Also, no whistling that I can detect.

Because I didn't write it down, I used a process of elimination from my carb calibration kit and have fitted the following rods and primary jets:

Rods: (.067" x .055") #1463 - 6755
Jets: (.083") #1422 - 120-383

The secondaries are unchanged from stock.

Alex

Hi Alex,

A better combination for the primaries would be
1463 rods with 086 jets The 083 with the 1463 rods would be a very lean combination.

Jim

Make sure the throttle return spring is strong enough. And that the cable is not binding?

jrv8 wrote:Hi Alex,

A better combination for the primaries would be
1463 rods with 086 jets The 083 with the 1463 rods would be a very lean combination.

Jim

Or leave the 83 jets in and bung in a set of 63-47 rods. (1443) It will run well on this setup but the cruise won't be quite as lean as the 86 jets with 67-55 rods.

The pilot screws and float heights are all over the place on a carb taken straight out of the box.....could be worth checking.

Thanks folks - I'll change the primaries to .086" to see what that does. Certainly one side of the car is running leaner than the other due to purple exhausts, so it's clearly running hotter - again, the mixture screws need playing with also to try and balance this out.

I have thought about upping the strength of the throttle return spring - it is not very strong which could explain the high idle.

I'll also look into the float height setting.

Anyway - one step at a time, primary jets first, then I'll update.

Alex

P.S. - Great wiki Pete - really useful - I have a shortcut to it on my desktop!

Alex,

Just to be different because I know it works and have set up two 3.5 engines with this combination.

Rods 1441 (.062 x.052) Jets 1421 (.080) secondaries as stock.

This gave a perfect A/F ratio at WOT right through the rev range and returns 28 MPG on cruise.

The engine is 3.5 with stage 1 heads and fast road cam Just dynoed at 208 BHP. and thats with block hugger manifolds on this engine.


Kevin.

CastleMGBV8 wrote:Alex,

Just to be different because I know it works and have set up two 3.5 engines with this combination.

Rods 1441 (.062 x.052) Jets 1421 (.080) secondaries as stock.

This gave a perfect A/F ratio at WOT right through the rev range and returns 28 MPG on cruise.

The engine is 3.5 with stage 1 heads and fast road cam Just dynoed at 208 BHP. and thats with block hugger manifolds on this engine.

Kevin.

Hi Kevin,

I'm not doubting you for one minute but I'm suprised that the combination you used worked as it is 19% leaner than stock on cruise and 21% leaner on the power step. (You sure that the jets weren't 83's? )

Just goes to show that you really have to experiment to see what works on your own engine!

v8alex wrote:Thanks folks - I'll change the primaries to .086" to see what that does. Certainly one side of the car is running leaner than the other due to purple exhausts, so it's clearly running hotter - again, the mixture screws need playing with also to try and balance this out.

I have thought about upping the strength of the throttle return spring - it is not very strong which could explain the high idle.

I'll also look into the float height setting.

Anyway - one step at a time, primary jets first, then I'll update.

Alex

P.S. - Great wiki Pete - really useful - I have a shortcut to it on my desktop!

Hi Alex,

The wiki is not mine, just those two articles, I'll tell my mate that you like it!

If one side of your engine is running hotter than the other there is not much you can do with regards to the carb. The manifold that you've got (same as mine) is arranged so that half the carb feeds two outer cylinders on one bank and the two inner cylinders on the other bank. The other half of the carb does the same thing with the remaining cylinders.

You know as a quick test you can leave the 83 jets in place and just bung in the rods that I suggested. You should at some point check the float height, they are usually miles out!

Pete

Pete,

Not suggesting that your alternative rod/jet combo won't do the job.

The primary jet was definately an .080 and although I cringe to say it, this is the recommended combo for a road modified 3.5 from RPI, it wasn't until my friend had his car on a rolling road and they checked the A/F ratio that we realised how good it actually is.

My car with the original 3.5 and this set up was very good under aceleration but had a slight hunting on a light throttle at low speed.

I now of course have the new 4.35L engine supertuned as per your recommendations, and it does run very well on the power of just trundling along.

Regards,

Kevin.

CastleMGBV8 wrote:Pete,

Not suggesting that your alternative rod/jet combo won't do the job.

The primary jet was definately an .080 and although I cringe to say it, this is the recommended combo for a road modified 3.5 from RPI, it wasn't until my friend had his car on a rolling road and they checked the A/F ratio that we realised how good it actually is.

My car with the original 3.5 and this set up was very good under aceleration but had a slight hunting on a light throttle at low speed.

I now of course have the new 4.35L engine supertuned as per your recommendations, and it does run very well on the power of just trundling along.

Regards,

Kevin.

Well you live and learn!

Maybe the block huggers were having some effect on the carburation, the lumps that I've worked on have all had fairly free flowing exhaust setups.

Glad to hear your motor is going well!

(What setup does your 4.35 run?)

Pete

v8alex wrote:The idle jets appear clean (the carb is brand new out of box), but I have checked anyway. Also, no whistling that I can detect.

Because I didn't write it down, I used a process of elimination from my carb calibration kit and have fitted the following rods and primary jets:

Rods: (.067" x .055") #1463 - 6755
Jets: (.083") #1422 - 120-383

The secondaries are unchanged from stock.

Alex

83 67/55 will give you 24.3% leaner than stock in cruise and 17.6% leaner in power.

That is quite lean and from my research and experimentation, I would consider it too lean.

You need to get richer but in small stages.

RPI seem to suggest using the 80 jets and 62/52 rods which give 19.3% leaner than stock in cruise and 21.2% leaner in power.

You can always try the 83 jet and 65/52 rod which gives 16% leaner than stock in cruise and 10.8% in power. A fair bit richer than you have at present - might be worth a try.

Are the idle jets screwed out at about the same amount? How many turns are they out?

Bugger - I don't have 86 jets.

So for now, I'll do a rod change to 63-47 and see how that goes with the 83 jets.

Weather permitting, I'll update tomorrow...

Alex