Charge tube / plenum volume ?

[rideblue]

Is it possible for the volume of the charge tube, plenum and intercooler to be too small to allow CV carbs to function properly? In other words, can the air space from the turbo to the carbs be too small, letting boost build quicker that the carbs can adjust for?
My carbs, BOV, charge tube, plenum, fuel system, turbo, etc. have no leaks and are in good working condition. It will run great up till about 1/3 throttle (7000rpm) makes 2-3 pounds of boost, and any more than 1/3 and boost will rise up to 8 PSI but sled runs like crap. EGA will indicate a lean missfire one time and than read rich the next time. EGT's drop when either happen. Main jets and needle clip changes have little or no effect on the condition. EGA reading done at the shop (8000 ft), and at parking lot (9500 ft), but not while riding at 10,000 - 12,000 ft.
I can unhook charge tube, rejet and install stock air box, and it runs great. So there should not be any electrical or mechanical issues causing this. Any ideas????????????

IHI RHB52 VF-4 spec turbo (1.8 l subaru), 45 PSI pump, Aeromotive fuel pressure reg, Bosch BOV (Saab)

 

[givemeaboost]

What size mains have you tried ? For that high of elevation I would think you're gonna need to be down to 120 or 122.5, Ken.

 

[JDKRXW]

Rideblue;
Let's see your set-up!

 

[joel0920]

once i have had similar problems on a bike,,
that time it was a small hole on one of the hoses to the carbs
the hole was so small that it almoast not was seen
but when i got pressure in the hoses the hole expanded and
made som pressure loss, that had a huge inpact on fuel..
sometimes the hole came to be bigger
sometimes small
sometimes not at all
i was comfused until i fund it

check all hoses for holes and be sure to use clamps at all ends...

a bad clamp ca make the hose to slip open at pressure..

good luck!

 

[peteracing]

RIDEBLUE, did you fabricated your aftercooler yourself?...

 

[rideblue]

For jetting i started off with what i ran before the turbo was added. 128.8 mains, 17.5 pilots and FS 3/4 turn out. Most guys with forced induction up here are running 125 mains. I have gone as big as 155 and as low as 115 for mains. started off with a 22 psi fuel pump, and installed a 45 psi pump last week and it was no help. I made the whole system myself with some help from a machinist freind. I will try to post pix, but last time I tried too, it would not work. It is not real good looking because i was planing on getting it working this winter and then making new header out of stainless and plenum out of aluminum this next summer. I pressure tested the plenum, but not the carb hoses. Will do that in the next few days.

 

[peteracing]

Rideblue, first, you only need about 3-5 psi of fuel pressure (plus boost)
and 2nd. did you built a "labyrinth" plate at your aftercooler exits to
carbs.? a straight exit from aft. core directly on the carb slides
makes the slides "jam" in carbs because of air flow(pressure) on them...
They stay half opened and carburation...

 

[rideblue]

Finally figured out the pix thing.

 

[Thomas800]

I know it's been preached time and time again, and it sounds like you already checked the carbs, but did you check REALLY closely on those carb tees. I had the same symptoms to a problem last year and was told to check the tees, which I did, and I missed a leak. I checked "everthing" else two or three times (even stripped the head down) before revisiting the tees and finding a leak on the bottom of the tees on the seam right between the carbs. Very hard to detect with it's location, but it was the problem. I machined some billet tee's and haven't looked back. I felt really stupid for missing it the first time, but I won't make that mistake again. Leaking tees will give you the same results as were mentioned by Joel0920. This may have no relevence to your problem but it never hurts to look. Good luck Tom

 

[peteracing]

Tomas 800 is right check that too. U can use soapy water on the "t"s
while running low boost with track off the ground...
Do yourself a favor and buy at least a aftercooler for your project.
Also, use PLENTY of thermal insul. on fuel tank.(just seen your picts.)

 

[sledheadgeorge]

Sounds like a leak to me as well. Its all about the plumbing.

 

[Tuco]

He has the MPI billet Tees and all new hoses.

 

[Thomas800]

What is the pitot tube shaped like inside of the plenum? Does it turn to the right side of the sled once inside the plenum? You may want to look at installing a pitot tube in the charge tube, before the 90 into the plenum. The smaller diameter of the charge tube would increase the velocity at the tip of the pitot tube and give more bowl pressure at boost. Just a thought Tom

 

[rideblue]

The base fuel pressure is set at 4 psi and the regulator raises 1:1 with boost. Checked it with a fuel pressure gauge, 12 psi at 8# boost. The pressure regulator is designed to be used when retrofitting a carb and turbo onto an efi engine running 45-70 psi of fuel pressure. Thought that it needed the bigger fuel pump to work right.

The carb vents come straight off of the plenum, no fittings inside. There is nothing else inside of the plenum, straight shot into the carbs. I will try to move the bowl vent line to the charge tube an see what that will do.

I have been thinking of a way to try to mark the slides to see if they are opening all of the way. Any ideas on that? I will try again to leak test my carbs and plumbing tonight.

I have a couple of intercoolers that will work, but question how effective they are when the hood is consantly covered with snow. They need cold air flow through them to work, without that, dont see how well it actually works in my application. I am a boondocker, not a hill climber, so my hood is always covered with snow. I am not bashing systems with intercoolers, just not sure how much effect they really have while boondocking. I am thinking about water-alcohol injection later on once my system is working better.

 

[rideblue]

Forgot one thing. The front of the fuel tank is coverd with tank insulation out of a Grizzly heat kit.

I had a customer with a WR426 a couple of years ago with the same problem. Checked jetting and the carb a million times, valve lash, ignition, compression and leak down. After a summers worth of frustration, turns out their is an o-ring gasket (not availible) that seals the top bottum halves of the the FCR carb. It had a small rip in it (from a backfire through the carb?) right where the fuel screw goes through both halve of the carb. That caused a small air leak into the pilot curcuit, but only when the carb got warm.