Jets etc for 40PHH carbs

[riceturbo]

I have now got my 40 PHH carbs on my 1600 and although in the main I'm happy running below 2000 revs is not smooth but there's no popping,coughin or banging. Nor can I get the tickover below 1200 revs (actual), I was told to use 65 pilot, 160 fuel, 190 air and 40 pump. With those in it just stunk of petrol and did not run smooth at any point. I have been running 57.5 pilot, 135 fuel, 150 air and 40 pump but I'm convinced this isn't ideal, there must be somebody out there as mad as me that has found a combination that works. The engines has had no mods at all and the carbs were brand new when I fitted them on a manifold meant for Webers but using insulators custom made for the job. Being in UK never helps but I'm known for doing things differently to the boring MG people.

[Gregs672000]

What size is your main venturi or choke? 32mm? Probably. You get a rough main jet number by multiplying that number by 4, so 130 main is a good start, then add 40 to that for your main air. Ballpark numbers. Your pilot is way too big, probably need a 52, or a 55 or so. I would guess 52 pilot, 130 main, 165 air. The air is for correcting over fueling at higher rpm.
How well do you know these carbs and how to tune them? Would you consider going and getting a wide band A/F set up?
If you're in the UK, contact Eurocarb.com for suggestions... good guys.

[david premo]

Greg, with all respect I think your recommendations should be on hold until a little is known more about his engine. It has been my experience that small displacement engine have lower velocity in the inner Venturi and tend to need large jets not smaller jets. It is also true that extremely large cam’s with long “duration” need bigger jets than the same engine with a milder cam. You loose a lot of suction coefficient with small displacement engines not to mention 40PHH carbs come in several versions, wide cover, narrow cover, type 2,3,&4 versions. Additionally the type 4 narrow cover have completely different inner ventui’s compaired to the wide cover carbs. It all ends up being an issue of the weight of the fuel vs the velocity of the air going through the throat of the carb.
Dave

[troyt]

IMO, Both Greg and Dave are correct. Greg is correct in giving you some sound starting numbers if you have no idea where to start. If you don't have access to a dyno, or have an onboard AFM system, then you've gotta have somewhere to start. However Dave is correct that these "starting point" suggestions have the possibility of being absolutely useless as every engine has it's own requirements. Cam profile, compression ratio, air filters/velocity stacks, engine condition (leaks, timing, valve clearance, etc) all effect the required jetting to get a set of sidedraft carbs working properly. Unless you have a large selection of main fuel, main air, pilot, and pump jets, along with a lot of time and patience, you are often better off going to an experienced pro to get your tuning done.

[riceturbo]

Quick update: The carbs are S5 type, filters are K&N (local company), 2" stacks. Standard compression and cam and good compression on all cylinders.

[Gregs672000]

Agree with all... and again I say... what... you all know... WIDEBAND!

Today I ran my Isky cam a bit more. Oh my, I do like it! But what is relevant here is that my air fuel ratios are dead on at cruise with this cam, where as they were off running the Racer Brown... that's because I jetted the carbs for the Isky before I had to swap it out last summer when I had rocker problems. But man this engine likes this cam!

[2mAn]

I really want to install a wideband soon... I know it’s recommended to install at the northern lost position but it seems like there is zero room for it, whereas mounting it horizontal opens up endless possibilities. Does it matter how at back it is because if that’s the case I can also think of a good spot for it

Hopefully this doesn’t derail and adds to the discussion

[Gregs672000]

I don't see why it would make any difference where it is in the exhaust so long as there are no leaks throwing off the reading. As long as the sensor has the ability to shed condensation it should be fine.

Truly, you will love it. Â couple days ago I came back home from a drive and noted it was idling at 13.1 or so, where as it had been in the 14s. Engine was rocking a bit. Turned each idle screw a 1/4 turn each way and monitored response with the engine and guage... richening and leaning did not solve the issue, and I could see the effect on the Guage and know it wasn't A/F ratio. Pulled off the horns for 2 and 3 and checked balance... front carb was slightly off, reading 2.5 while the other was 3 on the meter (love the snail meter BTW... miles better than a unisyn). Balanced back the flow, rechecked A/F ratio (back to mid 14s) and no more rocking. My carbs linkage is slightly modified to run the Dellortos smaller shafts, and it had slipped a tad. Fixed that issue. Later today I plan to do some hard power pulls while my wife calls out A/F numbers cause I think I'm overfueling at high rpms... only with a meter could you know. Then I can safely fine tune it with the air jet and not worry if I'm leaning out this high compression engine under high stress... priceless. Try reading plugs and doing that... not.

[riceturbo]

latest update after being battered by the authoroties for the shipping of bits of brassware I now have a setup that is the best its been since I put the Mikuni's on the car. Thanks go to Dave Premo for all his advice so far snd I am now running Pilot 60, Pump nozzle 50, Main fuel jet 140, main air jet 160, bleed pipe M. I know there may be minor adjustments to come but I have now a selection of jets to help me along the way.

I managed to get a quick session on a friends emissions analyser and the results were quite good all through the rev range admittedly with a few hiccups but hey she's 53 years old and the engine is still good and has never been rebuilt. The analyser he has only checks parameters necessary for our yearly MOT test and although my car is now exempt from the test he said it would pass even with the ever stricter regulations and I think that's not bad. We thought that we had a slight ignition timing problem so when I got home I set about rectifying that and eventually ended up with no sparks at all so at midnight I gave up and went to bed. I woke at 6 am and thought I had the answer, the last time we had a sudden loss of sparks it was the ballast coil so I was outliterally at the crack of dawn checking that out and no fault found so I decided it was something with my simple electronic ignition module and as a temporary measure I would return to points and condensor, only a two minute job but when I unplugged one of the connection the wire pulled out of the connector plug, new connector fitted and started first time on the electronic.

Over the years we would get an occasional spit but we never really bothered about it and with the erratic timing diagnosis I may have cured that problem as well.

I took her for a 20 mile drive and is now running a lot smoother while driving un 2k revs in top and is pulling a lot stronger from low revs' I know that I still have work but I now feel I'm heading in the right direction. All I have to do now is raise funds and get it in for a proper setting session with our local wizard but when heshakes his head and starts muttering about Mikuni jets I can present him with a good selection. Thank you again Dave for all your help in getting me going more efficiently down the right road

[CSP311]

At least I'm not the only one getting hammered by the UK import tax man... Good to see a SP owner here on the board from the UK.

[S Allen]

David Premo is definitely the man when it comes to Solex tuning and quite a few other things as well. Sounds like you have a fun driver now. Enjoy the ride.

Steve

[david premo]

Thanks, glad it helped.

[riceturbo]

Even when she's been a bit tempremental we still have fun, my motto has always been "if it isn't fun don't bother" and I'm now in my 22nd year of fun.