I've never had so much trouble with SUs before, please help!

[Bork Volvo]

Okay, so I completely built a new engine for my Volvo Amazon based on the B20 engine I have in my 1800S. Other than the Penta sourced B20 block, they are more or less identical builds. This is a combination that has served me well in the past, so I'm totally befuddled as to why I'm not getting good results:

Volvo Penta AQ130 (B20). This was a freshwater engine in healthy condition that I converted to street use. It had a rebuild one year prior to my ownership.
Head was shaved for 10.5:1 Compression
B20B headgasket
Isky VV71 camshaft
Isky high rev lifter kit
IPD HD oil pump
OEM Pierburg mechanical fuel pump

Timing gears checked. Timing checked with a strobe: currently set at 10BTDC idle (800 RPM), 34BTDC at 3,000 RPM
Valve lash set to .017 hot.
Compression was tested, all of the cylinders were around 170PSI, give or take 2lbs each.

SU HS6 carbs, completely rebuilt with new springs, dampers, throttle shafts, gaskets, floats, jets, etc. I am using KD needles and MMO for dashpot oil like I always have in all my SU carbs. Chokes are set and fully functional. Linkage checked and has full extension, it doesn't bind. Throttle will open completely and does not bind. Dashpots rise and fall at the same rate. Carbs were balanced independently and verified with a traditional Unisyn. No air leaks or excessive PCV pressure.

I am using the same Bosch mechanical advance distributor as my 1800, with Pertronix ignition. NGK straight copper plugs with IPD silicone leads.

Resurfaced flywheel, new clutch and pressure plate, M41 in tip top condition. New AGM battery, new Bosch blue coil.

I have a fresh tank of 93 Octane (AKI).

My problem is that at 4,000RPM, it's like my car hits a wall and won't accelerate any further. Adjusting timing, mixture, etc, I still seem to hit this artificial wall at 4,000RPM. In any gear? 4,000RPM. Neutral? 4,000RPM. Slow application of throttle? 4,000RPM. Even if I pull the choke at 4,000RPM, it still won't go beyond. I swapped out the distributor with an identical one set up for points, the wall was now at 3,500RPM. I replaced it with the Pertronix setup and I was again at the 4,000RPM wall.

It seems like nothing I have done will get me past this wall. It revs up fine and feels completely normal, then just stops at 4K. It's very unusual. The 123GT tachometer is within spec, so this isn't a false reading. Foot on the throttle or even disconnecting the linkage and moving it by hand, no change. I thought about buying a Gunson Colortune to check if it's running lean or rich, but they are all out of the 14mm size I need and I cannot find them on the secondhand market. The only thing I can think about is adding a wideband air/fuel meter and measuring it with an O2 sensor, but I don't have money for that right now.

With my previous B20 build in the 1800, I have no trouble getting up to 7,000RPM, and it still pulls really hard around 5,000RPM.


Your ideas and advice are appreciated, thanks in advance!

[BorderVolvo]

A friend had a similar problem (wasn't a Volvo) which turned out to be a partially blocked exhaust.......was a fuel injected engine.
It wouldn't even rev up passed 5k when stationary, split the exhaust at front pipe and away she went.
Maybe worth checking?

[Bork Volvo]

Originally Posted by BorderVolvo

A friend had a similar problem (wasn't a Volvo) which turned out to be a partially blocked exhaust.......was a fuel injected engine.
It wouldn't even rev up passed 5k when stationary, split the exhaust at front pipe and away she went.
Maybe worth checking?

Thanks for the tip. The exhaust is recent production exhaust from VP Autoparts or CVR, I forget. I am using a B20 cast iron manifold with a 2-1 downpipe. I'll see if disconnecting it after the downpipe makes a difference, I know they have some less restrictive sport models available that could improve breathing.

[Groundes-Peace]

This sounds to me like your electronic ignition.
I am not familiar with the one you have but, I had a similar issue with my 123 Tune ignition when first fitted.
My car was limited to 5000 rpm.
With assistance from the manufacturer's help line my car will now run to 7000rpm.

[Bork Volvo]

Originally Posted by Groundes-Peace

This sounds to me like your electronic ignition.
I am not familiar with the one you have but, I had a similar issue with my 123 Tune ignition when first fitted.
My car was limited to 5000 rpm.
With assistance from the manufacturer's help line my car will now run to 7000rpm.

Thanks Groundes-Peace,

I thought that was the case at first, so I tried to rule it out by installing a stock points ignition with new cap, rotor, condenser, and points, but it actually reduced max RPM at WOT to 3,500. FWIW, my 1800S has the same exact Pertronix and coil setup and it was totally plug and play.

I am very interested in a 123 ignition because I have only heard great things, but I cannot afford one right now.

[Rustinmotion]

Just to be certain remove vacuum advance and plug manifold end, any difference?
Will it rev past 4k with no load if yes it's likely fuel or a blockage of intake or exhaust if no it's likely ignition related
Other thing to try is drive the car trying to get it past 4k and do an ignition cut that's ignition off and clutch in at same time and coast to. a stop, pull the ugs and see if its rich lean or ok, sounds like fuel starvation so it should rev higher under no load and the plugs should be light grey to white

[Derek UK]

Something to check as you say you have fitted new dampers to the SU's. It is possible to use dampers with a bleed hole in the top if there isn't a hole in the dash pot casting just below the threaded hole for the damper. You mustn't have a hole in both or you wont have enough vacuum to raise the piston sufficiently. It may raise it about half way and this could be the reason you only get about 4k revs. Early dampers have a brass top, usually with a hole. Later ones have a black plastic top and I think these don't have a hole. If that is the case there should be a hole just below the threaded area.
There is also the possibility that the dash pot and pistons have been swapped over and are now mismatched. Easy to do during a rebuild. They come as a pair and must be kept together.

[Ron Kwas]

Bork;

First, compliments on a very complete listing of the preconditions, although some are not likely to affect the ability to rev past 4K...

Also, your title might be a bit on the presumptuous side...I don't think you can point the finger of suspicion squarely at the SUs just yet...

Ignition is one of the first things which is likely to hold back the revs, but with idle and advance checked and apparently right, I would move on to what BV, also mentioned, and the second thing I thought of: Breathing...I presume you are doing your initial tuning and checks without Air Filters installed, so it can't be blocked AFs...so what's left is the Exhaust Sys...I'd disconnect and make a road test run with an open sys ...preferably not down your own residential neighborhood!

By "Dashpots rise and fall at the same rate." I presume you mean between the two carbs, not for each carb...Dashpot Rise must be dampened, but Fall must not! Also, are you doing your tests only in the driveway (not road/under load), in which case I must ask if your initial mixture setting after carb rebuild was at 12 faces down on the Jets? I don't know this for certain, having never done this, but if, for instance, you could get the engine started with them flush at the Venturis (with the help of ether I suppose), the engine might be able to be reved against its own inertia, but it would clearly be very lean (and not so good for it!), but it wouldn't be able to make much power against a full load, and I wonder where it would "run out of breath"...

Good Hunting!

[Burdekin]

Çheck the timing at 4000rpm. How old is the coil?

If a fuel delivery issue things to check; fuel pump, bowls float correct height, no holes in floats and carb fuel intake valve operational.

[Burdekin]

As per Ron's suggestion, how did you set the fuel mixture? A O2 wideband sensor and gauge will help check what it is at higher revs when driving.