VH500 running rich

[dennis.t]

My 1955 VH500 alloy head with a standard Monobloc set up is running very rich. It is hard to start and when it does the plug fouls with black soot. Black smoke comes out of the exhaust.
Using the Amal Monobloc guide I put the needle down one notch then two with very little improvement. I changed the main jet from 200 to 140 (one I had lying around) and that improved both starting and running but still too rich. I changed the pilot jet from 30 to 25 with no apparent improvement.
I am thinking of changing the needle jet from 106 to 105 but will that make much difference?
For more complete information the Monobloc is a used item but has been refurbished by me with a new needle, jets and jet holder.
The pistons looks domed to me and may be a high compression one. A compression test yesterday gave a reading of 150 psi.
When I can get the bike to run, it revs and ticks over and responds to the choke and the magneto advance but uses a lot of fuel, so I am thinking that this is a carburation problem. I checked the fuel level in the float chamber and that is correct and the float needle is operating correctly.
Any suggestions are welcome.
Dennis

[charvey]

Hi Dennis I had the same problem but I noticed that the choke slide was not going right up when on full throttle even at half throttle it was slightly down so I was running on half choke all the time I have now moved the choke lever round to make sure the cable is positioned correctly and has not slipped out and have shortened the inner cable so there is no free play and the cable cannot slip out, I thought I was using a lot of petrol when I last went on a long run this may not be your problem but It might be.
The level in the float chamber should be below the main jet there was some info on the web site a while ago I check mine with the top of the float chamber off and did resurface the float needle with some brass cleaner to make a good fit and make sure it cut off hope you sort it Clive.

[dennis.t]

Clive,
Thank you for your suggestions. I did not want to list all of the checks I made as it is a long list. However, I have checked the choke action so I can rule that out. I checked the fuel level in the float chamber using a clear plastic tube over the exposed pilot jet. According to some sources the fuel level should coincide with the 'pip' on the float chamber cover beneath the word 'Amal' and it does.
One thing I have not checked yet is whether or not the new needle I purchased is correct. I found a reference today giving the needle type as 'C' with five rings and a length of 3 and 13/16 inches. I will check the needle tomorrow and even if it looks correct I will try an old needle in its place.
Dennis

[simon.holyfield]

I had a similar problem with my W/NG. If you haven't already, get a new needle and needle jet from somewhere reliable, like Burlen or Hitchcocks.

The needle only has to wear a small amount to make a huge difference to the mixture.

[dave.owen]

Has the carb been fully stripped and cleaned on a donic cleaner if possible. Check all drilling for blockage.

[dennis.t]

Simes and Dave,
Many thanks for your replies. I will probably get a new needle and jets from a reputable supplier if only to keep on eliminating potential problems. I have no sonic cleaner but I checked all airways during a full strip down of the second hand carb. That carb certainly works better than the one it replaced apart from the rich mixture issue.
Dennis

[will_curry]

Dennis,

Do you have an air cleaner fitted?

[ian williams]

my NH was running rich and had an off idle flat spot , i tried needle positions and changing the slide which made little difference , pilot jet had to remain same otherwise i didnt have the adjustment range .
i changed needle jet from 106 to 105 and it now runs somewhere near how it should, so yes if its rich knock the needle jet size down one size .
i did try different main jets and think i am on a 210 at present but have gone between 200 and 230

i have new burlen monobloc carb and have alloy head .

presently using old lodge HBLN plugs but also have champion N5 i use as well at times , also set to 5/8" on timing .

have been running mine on E5 as well as on 100LL avgas and runs same on both .

cheers
ian

[Brian_Walker]

Published details for Ariel 497cc VH hunter 1955-59
Monibloc 376/11
Bore 1 1/16"
Pilot 30
Cutaway 3.5
Main Jet 200
Needle Jet 106
Position 3
I think C is the correct needle.

I have found issues with the new brass float needles as they are a little heavy and may fail to seal properly when runing causing a intermitent high float level (can be checked with a vacuum leak test on the petrol intake hose to the carb). Could also check the float is not fouling on the cover. I made a perspex cover for mine to check what was happening. As you know float height is adjusted with shim washers under the float needle seat. Sometimes on older carbys there is no way to lower the seat any further without reducing the length of the float seat by turning it down on a lathe. My old carb had a brass float so I was able to put a dab of solder on the float to get a correct height.
Help this helps you shouldn't need change jet sizes or anything else, worst case I have seen an grossly incorrect needle (not a huge difference between a C and a D) however this was a special Norton one someone had thought was a standard monobloc needle however it was a lot shorter and only had 3 notches instead of 5.
Hope this helps. You did say NH in the text and if that is what you want its a 376/8 that has a 25 pilot instaed of 30 otherwise the same.

[nevhunter]

Monoblocs weren't regarded very highly when they first came out. The Later Concentric was far better and used on quite a few Speedway Jap's etc Disregard if not relevant to your problem. Nev