SwedeSpeed - Volvo Performance Forum banner

1 - 1 of 1 Posts

101 Posts
Discussion Starter #1 (Edited)
Most of the classic Volvo folks have plenty of experience with SU’s. I have two cars with B20s and have never never even set eyes on an SU. Seems there have been a few questions on other carb options so I thought I would share my experiences.

Mid-summer I purchased a ’69 142 in a moment of weakness. It is a bit rough around the edges but I enjoyed how it drove and it came with a trunk full of goodies that made it seem like a deal. The original carbs were removed and replaced with a Weber DGES 38 and a set of 4-1 headers. Initially the car barely moved down the road, but with a little bit of tuning and a new fuel pump the engine ran reliably and had a surprising amount of power. The big problem was a significant lean stumble at cruise. This seems to be a common problem with the DGES 38. There are plenty of discussions out there on this problem from drivers of jeeps, Opels etc. Some blame the idle circuit, others the e-tubes. I got sick of messing around with it an opted to try the one of the prizes that came in the trunk. A set of DCOE45s.

I did not expect to gain any power from the DCOEs but the intake noise and look were impossible to pass up. I installed a wide band sensor with the carbs and got about tuning. It is a slow process especially when you have to wait several days when you order up a new set of jets. After a few weeks I got them tuned reasonably well. Almost all driving conditions gave me an AFR between 12 and 14. Not bad! However I was still plagued by the occasional intake backfire. AFR readings were spot on, timing was checked, advanced and double checked. Power was way down and throttle response was sluggish. I figure these bad boys were just not a good match for the otherwise tired stock engine. Not enough intake velocity and possible reversion. They got tucked away in the basement for a future hot build.

Back to the DGES 38, determined to fix the lean stumble. Now armed with a wideband I also discovered it was richer than Bill Gates at WOT. In fact I am running rich on either side of the stumble. I am confident the problem is in the transition from the idle circuit to the main circuit. As the idle circuit was already on the lean side I worked on the main circuit. The DGES has a power valve that opens to deliver more fuel when the vacuum levels drop. I blocked the vacuum port so it always delivered fuel to the mains. This actually helped quire a bit, no longer a stumble but more of a flat spot. Of course the fat top end was still there.

There are two opposite lines of thought in regards to the main air bleed. Weber documentation says it only affects top end ratio but DCOE guru Keith Franck claims it mainly controls when the main circuit comes on. I tested various bleed levels by dropping in a large bleed jet and further restricted it with various sizes of wire. Not a permanent setting but a great way of quickly performing tests. I found out that the air bleed size affects both when the mains come on and the top end ratio. A smaller air bleed helps fill out the lower end a bit but also richens the top end quite a bit. Fix one problem make the other worse. I tried a smaller main jet. It did the same thing, helped lean out the top end but made the transition stumble worse. I have ordered an F5 emulsion tube. From what I can tell from the magical world of emulsion tubes this bad boy should give me a richer low end and might not drown the top end with gas. If that doesn’t work I am pretty much at the end of my rope. Perhaps a less restrictive exhaust would lean out the top end? This brings us to carby #3.

When I bought my ’66 wagon it came with dual Mikuni HSR 42s, presumably source from he who shall no longer be mentioned. After balancing the two and fixing other various engine problems the very tired B18 purred like a kitten. Throttle response is crisp to the point where I started getting annoyed with the previously imperceptible turbo lag on my MKV GTI. All was good until I opened up the restrictive tiny exhaust system with a 2” under axle job. Lost power at high RPMs. On went the wideband sensor. Found out I was lean at the low end, rich at the high end. Put the needle at its highest setting, bigger main jet, smaller idle. On the road everything drives well, crisp throttle response, pulls well from idle to redline. Wideband shows that I am running rather fat at cruise, too much overlap between main and idle circuits. I can cure this by dropping down the needle but this causes a lean spot under light acceleration. I need to order a bigger needle jet so I can lower the needle and have a good mixture under moderate acceleration.

I only have two complaints with the Mikunis. First of all they need a high idle. The idle circuit delivers a lot more fuel at higher rpms. If the engine bogs down for any reason things get lean and can easily stall. You can run the circuit rich to drop the revs but then things run real rich at cruise. 1100 rpm idle it is then. Problem 2 is with the throttle linkage and the soft mount. Hit the throttle stops on the carbs real hard and you can push the carb off the mount or even worse cause the linkage to stick at WOT. I have solved this by putting a linkage between the two throttle shafts to stop them from spreading apart, right now it is a simple zip tie, will get around to a metal and bearing solution over the winter. Fuel economy isn't so good but I have a heavy foot and things should get better when I up the needle jet. All in all I am delighted with the Mikunis and may put them on the 142 if the DGES problems cannot be solved. There is no need to source them from potentially questionable suppliers. Some easily sourced linkage bits and some new jets as described above is all that is needed. Of course there is the option of going back to SUs, not sure if I want to learn yet another carby system. May just go FI if it comes to that.
1 - 1 of 1 Posts