Backfire through Weber Carbs Update.

[gzucca]

Here is the update on my restoration project and the problem with the carburator lean backfire issue. I suspected that the isolators were leaking being that I could see cracks in the rubber when I leaned on them. I also noticed a trace of smoke at the base of the carbs when they backfired. I replace the isolators, but while I had everthing apart I did a real bang up job polishing the intake manifold. I sanded it down with a air powered Dynafile to level the rough castings, progressively sanded it to P600 with a DA and hand sanding, then polished it on the buffer. Results are very good.

Put everthing together and the problem was sill present. The smoke I notice actually comes out of the base of the carbs by the accelorator pumps, Their is an air vent there that will release pressure if carb backfires.
Did a little more checking and figured out that the intake cam was set wrong. In my infinite wisdom I thought that this engine had the late US cam caps that were adjusted for US emissions so I did the hot rod fix and advanced the intake cam 5 degrees. Well I was wrong this engine had the non US caps that did not require the adjustment. I fixed the cams and most of the lean backfire issues went away.

The next thing that I discovered with this engine is that it never really heats up and I am noticing water accumulating in the oil. I checked the thermostat and found that it is a 160 degree unit. I always ran 175 in my Spica motor and it worked very well plus it puts the needle right in the middle of the gauge exactly where I like to see it. Is there any reason to run a 45mm carb motor at 160 degree? It seems to run fine at 180 and the oil is clearing out.

After futher running of the motor I notice fuel gathering in the front carb and spilling out the air horn. I decided to check the float levels and found that the floats were set to 5mm. The Weber spec on a 45MM DCOE is 8.5mm. I bought this motor several years ago and it was supose to be a properly set up motor. Why would the previous owner set up the floats to 4mm instead of 8.5mm? I now question everthing else about the motor. After reseting the float levels I believe that I solved the flooding problem. I haven't noticed and excess fuel in any of the chambers.

Right now the motor runs pretty good. Still have a slight issue with lean backfire at idle. High end feels real good. When I race the motor it doesn't want to come down to slow idle right away. It hangs around 2000rpm for a while then settles down. Any idea what I should check next? Carbs are balanced.

Any help would be appreciated

Glenn
Restored 74 GTV

[Gordon Raymond]

Hi Glen,
Is the photo as the engine is now set up? If so you are missing a very important support bracket and plate that prevents the carbs from bobbing around on the rubber isolators. Without the support rod from the engine mount and plate to keep the carbs in line. there is no way they will stay balanced as they move around from running vibration. Webers do not like to be shaken, and any imbalance will cause slow return to idle and possibly some popping as well. Did you set both the float drop and level? 8.5 mm level and 15 mm drop. Do you have a FRB 11 filter and pressure regulator in there somewhere? What kind of fuel pump, mechanical or electric. If electric, not more than 3-4 psi. pressure. The FRB 11 (FISPA) will even out the fuel pulses. Just some ideas to consider. Gordon Raymond

[gzucca]

Hi Gordon,

Thanks for the reply. I do have the bracket on the carbs. It was not on there initially but it is now and it definity helps with the bouncing. I set the float level to 8.5mm exactly but I did not check the drop (15MM). How important is that? I can't imagine it will cause a problem unless it is bouncing. I guess I should check that as well.

I have a facet electric fuel pump and there are two filters in line. The original spica filter near the tank and a generic one next to the carbs. The fuel supply is very clean and appears to be delivering 3.5 psi and very constant.

My concern is that the original owner of this engine was running it with a float level of 4MM. It was just pouring gas into the motor. I am concerned that the idle jets are incorrect as well.

I am going to go back an get check the TDC with a gauge. Set intake and exhaust cam exactly. Work on ignition timing and check advance. Then go back and try to set up carbs for good idle.

At first it appeared that my idle mixture screws had no effect. That makese sense if the float levels were too high, it was just pumping fuel into the motor. Now that is fixed I should have better control with the mixture screws.

I have one of the original fuel regulator/filter units. Is it a good idea to use that on this motor?

Thanks for your help.

Glenn

[Gordon Raymond]

Yes Glenn, the FRB 11 is a very good unit, it's ball / spring + diaphragm does a good job of protecting the float needle valve lever from wear. It was not only used on Alfas with Webers, but all Ferraris of the era as well, both street and competition cars.
As for jetting, and some phone advice in your area, call Gronway Perry. Gron is a DCOE master, and has all the files for about any application. He lives near San Jose. His cell phone # is 408-781-3770. Leave a message and feel free to use my name. He has helped me out on a few tough ones. Gordon Raymond

[ill_will]

what jetting and chokes are you running, and what has been done to the motor (if anything)? I did a bit of reading when i was sorting my carbs out and think I remember someone saying that slow return to idle can be caused by the throttle butterflies being too far open at idle, exposing part of the progression holes. I guess this can be a 'bodge' to get the car to idle at the right speed when the timing/mixture isn't right.

Are your throttle butterflies all lined up perfectly?

Someone else said that webers are well known for lean 'backfiring' when cold - i guess you might find once you sort the temperature problem out it sorts it out. Water in the oil sounds messy. Do you know if it's there from a previous rebuild or is the head gasket going? Might be worth doing a compression test.

Good luck!

will