Tunnel ram carbs

[Mikemuir]

Hi there everyone just after a bit of advice from any of you Holley carb
Tuning gurus!!i have a 351 with street/race cam and 2x 390 vacuum secondary carbs . They are supposed to be tuned for tunnel ram 2x4 barrell but seem to have a very rich idle (burning eyes)stable and good idle though,and I have a bog/ hesitation off from light throttle and sometimes backfire through carbs. Any suggestions or should I be swapping out metering blocks for more tuneable quickfuels?

[solarite_guy]

Couple things:

1. Need a little bit more information about the motor.

• Is it still 351cid?
• Which cylinder heads?
• What is the CR?
• Do you have more specifics on the cam? Do you have the card?
• What us the initial ignition timing?
• What is the total timing?
• At what rpm does it reach total timing?
• Which intake is it?
• What is the manifold vacuum at idle?
• What is the idle speed?

2. About the carbs:

• What are the list numbers?
• How far out is each idle mixture screw? (Number of turns)
• How far open are the primary throttle blades? (At this point, how turns are in the idle speed screws?)
• Have you checked the float levels?
• What is the fuel pressure?
• Which color accelerator pump cams?
• At idle is there slack in the accelerator pump linkage of either carb?

Cheers

[Mikemuir]

Yes it is still 351.Maybe been bored 30thou over
Fully recond with receipts from previous owner
It has 2v heads
Not sure of CR
Cam is COMP cam Part no 32-225-4 Cam grind no 280H
I have set timing to 16degree BTDC
Total 34degrees
Running MSD
I think it reaches total advance around 2500RPM but will check tomorrow
It has a REDLINE manifold
Vacuum at idle 1000rpm is 8

Forgot to mention that I have put 50cc accelerator pumps on carbs
Running brown cams on number 2 hole
accelerator pump squirts at first move of throttle
Have checked fuel bowl levels all ok
Fuel pressure 6 psi through regulator from electric fuel pump
Holley carb part 0-8007
Idle adjusting screw on linkage approx 2 turns out
( my next check was to check transition slot by removing carbs)
idle mixture screws about 1.5 turns out

It runs really good when you are a little bit more aggressive on the throttle and seems to enjoy the accelerator pump squirts
If you slowly accelerate it feels that it takes a while to get the fuel flowing from the carb which is where the bog/hesitation and backfire occurs (possible lean mixture?)
When it runs on WOT at higher revs it loves the tunnel ram and i can see vacuum secondaries opening .
Let me know your thoughts?
cheers

[solarite_guy]

Thank you.

I would expect a a bit more manifold vacuum with that combination. Something over 10.

Have you tried spraying carb cleaner at:

• The base of the carbs
• Where the plenum and runners meet
• Where the intake meets the heads?

Check for a change in idle speed.

Do you know how the cam was degreed in? The ICL should be 106.

Try these tests 1 at a time. You may want to clean the plugs first.

Knowing the CR would help a little bit, but for an easy shot in the dark test, have you tried initial ignition timing at 18*?

The rich at idle could be a transition slot issue. While running that check, even it the setting looks right, try taking 1/2 turn out of the primaries throttle blades settings and put 1/2 turn into the secondaries.

Tuning with the accelerator pump cams can be tedious, but really not time consuming. You will know quickly if a change helps or hurts. Also, depending on the combination, what may seem obvious or is recommended isn't always true.

After you have tried the 2 tests above try position 1 with the accelerator pump cam. Remember to re-adjust the accelerator pump linkages.

Do the carbs have stock squirters?

It is possible a couple things are going on here. My biggest concern is manifold vacuum though.

Cheers.

[Mikemuir]

Thanks for all your help.
I have sprayed carb clean around the places listed and no change in idle.
I have already removed the main manifold and fitted new gaskets recently because I wanted to eliminate a possible area for a manifold vacuum leak.
One thing i have noticed is that when i remove the vacuum gauge from its port and leave it open the idle does run faster.
This perhaps indicating that there is no vacuum leak??
I have advanced the timing to 18 degrees
I have vacuum at 9.0 at 1000rpm
I will pull the carbs off tomorrow and check/set transition slot.
The carbs have standard squirters.
I am hoping that the cam was installed correctly and believe this was done by the engine recond shop so guessing it should be ok?

I will pull the carbs off tomorrow and report back.
Cheers for all your knowledge

[solarite_guy]

When checking the transition slot and try the above mentioned test, dial 3/4 turn into the secondaries throttle blade position when you pull 1/2 turn out of the primaries.

Once it fires you may need to tweak the primary idle speed adjustment a little.

Check the accelerator pump linkage after this as well.

[dunga]

What hg power valves are fitted with that low vacuum they maybe starting to open

[solarite_guy]

Good question. That was actually my first thought as well.

If the carbs are stock, they are 6.5s

[Mikemuir]

Took the carbs off and checked transition slot setting
Looked good
added 3/4 turn to secondaries
Sorry just saw the posts regarding power valves
I will grab 2 x 4.5 power valves today

Really appreciate your guys help
cheers

[solarite_guy]

6.5s should not be causing the idle issues.

I wouldn't put 4.5s in

Besides, your vacuum should still be more than a point higher than it is.

The 4.5s may cause a lean spot on throttle tip in.

[Mikemuir]

ok I will hold off on installing powervalves.
What else could be causing the low vacuum?
I have had the motor compression tested and think they were all pretty much around the 150 mark.
I have removed vacuum lines on the carb and installed caps but still only running 9.0 on vacuum

**** weather today so haven't been able to take it out to test the new changes.
1/cleaned plugs
2/added 3/4 to secondary throttle plates
3/changed accelerator pump cam to position 1

once again noticed that if I remove vacuum gauge and leave port open.the motor seems to be a lot crisper(only tested while parked up for a short period).
I didn't want to hurt the motor by running lean?

Let me know if there is any thing else I can check why it is parked up.

Cheers

[solarite_guy]

Your cylinder pressure is lower than I would expect.

Can you contact the shop that put the engine together and find out about the CR and if the cam was actually dialed in or just installed? Theoretically, installing it "straight up" will put it +4* giving you 106 ICL, but it is not uncommon for the pin hole to be drilled wrong.

Did you notice any changes in how it drives after moving initial timing to 18*?

Let's try the changes mentioned above before making other changes, so we don't lose track of what may need to be backed out.

Cheers

[Mikemuir]

Unfortunately I have inherited this motor but have had it confirmed that it has had a full rebuild
I also have all the receipts for the engine work.(2004)
I have been told that this was the cam that was installed but cannot confirm.
I don't think this engine has ever been dialled in with this tunnel ram set up correctly. This project had been sitting for a while and the car never used.
I have revived it and done a lot of panel work to get it back on the road.
The engine /c4/4.11 9inch diff has been installed by others.
I have checked over most of the mechanicals and it seems ok.
I have changed over the valve springs because they were a bit light, to match the assumed cam.
It did have 2 x vac 600 on it when i first got it ,but thought i should start fresh with some new 390's when i was having problems getting them to run properly.

Just a bit of background information FYI

[Mikemuir]

Here are my findings from the adjustments.

Engine runs a lot better on light throttle
On light throttle very small surge but once at about 2000rpm feels like its hit its power band and wants to take off!!!
If i push the accelerator to about 1/4 throttle the engine dies.sometimes backfire through carb and then fires up again once I have backed the throttle off.
I have tried putting the accelerator cam back to position 2 but didn't make too much of a difference.
I feel we are heading in the right direction
The car is a lot more drivable at light throttle
Any other changes I could make to remedy this bog at about 1/4 or more throttle?
Thanks for all your help .
Vacuum is probably closer to 10 at idle at about 1000rpm and steady.
Still probably a bit low??
cheers

[solarite_guy]

Alright, good news.

Unfortunately, I am going to ask you to spend some money now.

However, first is it at all possible to describe any differences between position position 1 and 2 of the cam?

Can you get the cam kit and secondary spring kit? Also, the next 2 sizes up on the squirters?

I would like to get your vacuum above 10, but let's test the accelerator pump possibilities a little bit first.

Cheers mate.

[Mikemuir]

Thanks for quick response.
I do have a variety of cams for the carbs.
Pretty sure the 50cc accelerator pump upgrade only came with yellow and brown cams but i do have quite a few of the others that I had for the 600's
I don't have the secondary springs but can get some.
I think I also have some larger squirters which i was also trying on the 600's when they were fitted.
Couldn't really tell much difference between setting 1 or 2
Which change do you think i should make first.
cheers

[solarite_guy]

Alright, please identify which cams, springs and squirters you have available.

Also, which squirters are currently in the carbs.

Cheers

[Mikemuir]

Hi there
I can buy the holley secondary spring kit (hopefully today)
The carb has factory squirters installed # 25
I do have a set of #35
I have Browm and yellow cams that came with 50cc upgrade and I also have standard orange cam original
I will also buy a cam tune kit
What size squirters should I invest in because I will grab them as well

Thanks

[solarite_guy]

For testing purposes get a pair of #27s for now.

For now leave the stock squirters and cam. Put the next strongest springs in the secondary diaphragms.

Let's see what that does.

Cheers

[Mikemuir]

All good
Thanks for that .going out shopping soon
Did you mean to go back to original orange cam or stay with the brown

cheers

[solarite_guy]

Stay with the one you have for now

[Mikemuir]

Swapped out springs to brown
You are the Holley guru!!
Running a lot better
Still a bit of a stumble and backfire under harder acceleration
Stumble and backfire through carb seems to happen when secondaries are starting to open(I can see them through the hole in the bonnet)
The transition under light throttle is now a bit smoother but still feel the secondaries kick in.
Should I swap out the spring to black or work on the accelerator shot(cams and squirter)
You have made my day and really really appreciate your advice.

Cheers

[solarite_guy]

Alright good.
In the end I want to get you to softer springs in the back, if possible. But leave it where it is for now.
It seems you are not stumbling right off idle so try the brown and blue cams. IIRC brown has 1 position. Blue has 2. When you try blue, start off with position 2.

Hold on to the 27 squirter for now.

Is the vacuum still around 10?

Cheers

[Mikemuir]

Just picked up the cams today in my travels so will report back soon, once i get a chance to take it out for a spin.
Noticed after last test drive that when i punch the accelerator it goes well for short period and then bogs /backfires through carb.
Guessing it doesn't have enough fuel squirting to get it through the transition into the secondaries ?
Hopefully that more aggressive cam will help
I was running the brown cam.
Still running steady 10 on vacuum

Cheers

[dunga]

Originally Posted by Mikemuir

Just picked up the cams today in my travels so will report back soon, once i get a chance to take it out for a spin.
Noticed after last test drive that when i punch the accelerator it goes well for short period and then bogs /backfires through carb.
Guessing it doesn't have enough fuel squirting to get it through the transition into the secondaries ?
Hopefully that more aggressive cam will help
I was running the brown cam.
Still running steady 10 on vacuum

Cheers

what do you class as a short period .

[Mikemuir]

Hi there
Sorry short period is split second and then bog/backfire

cheers

[solarite_guy]

It is when the secondaries open. It goes lean.

Do you have a vacuum gauge to hook up and read what the vacuum is when it backfires?

Want to suggest going up 2 sizes on the front jets and change rear plate to go up 2 sizes.

[dunga]

What does the fuel pressure do when all this is happening can you explain how your fuel system is set up pump type etc

[Mikemuir]

Fuel system is all good.
I have black electric holly and regulator set to 6 psi.
I have a gauge on the fuel line to the carbs and it stays steady at 6 psi
I have also been watching that through the hole in the bonnet and no changes in the 6 psi throughout the revs and when bogging.
Haven't had a chance to try the blue cam out on the road yet but will report back soon
I will also hook up vacuum gauge so that I can read that as well next road test
Cheers

[Mikemuir]

Managed to sneak some time out of work and have done a few test runs.
Blue cam didn’t make too much difference.
Hooked up vacuum and noted that hesitation/bog and backfire through carb all happening around 2 or less vacuum
If I keep vacuum in the 10’s when accelerating the motor runs sweet.
Really do think that I need to be looking at low vacuum problem because this is where all the problems are occurring .
Even on 1/4 throttle vacuum is dropping below 4
Maybe I should be checking lobe lift on cam next to confirm cam is ok?
Next step I would think would be to check cam timing
One other thing I noticed when I put new inlet gaskets in was that there was no valley tray fitted . Is this a problem?
I have now gone through a can of carb clean so believe I can rule out any vacuum leaks around the top of manifold and carb.
What are your thoughts?

Cheers