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Al Eckstadt
Al Eckstadt
Jordan, NY
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#825669 10/05/20 3:16 pm
Joined: May 2016
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S
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My Bro's Venom wont pull past seventy in top. It seems not to rev out. Dropping a gear makes no difference. It has a 30mm Concentric and 19 Tooth gearbox sprocket. he has had it down and retimed the valves which seem OK. Any thoughts on this?

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Either it is way over geared or the motor is sick. A stock MSS is good for 75 to 80 and they have lower compression and milder cam.


Laurence Luce
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Well. It's a bit of a 'how long is a piece of string' question!
We need more information I think. What is the performance like up to the 70mph? Is it crisp or sluggish? Does it rev out well in the lower gears?
Is it an easy (for a Velo) starter? If those are okay, we can assume you have a good spark, -a good magneto, so it's likely a carb problem.

We would normally start to go through the carb settings to track the problem down, for example if performance is good at lower speeds, you'd want to start trying different main jets. You need to do a 'plug-chop' to see the plug colour at WOT for a start. That'll give you an idea of the mixture at WOT.
You need a handful of new plugs.

Start with that.

(WOT=wide open throttle

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I'll get back with more info. Thanks for reply.

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Bry Offline
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Is this a recent problem, did he previously have it running ok?

Good advice given above on the carb set up.

I had a similar issue with my Venom, turned out to be the repro fishtail silencer restricting performance. I could not find an original fishtail at the time so got an extended drill bit and drilled a series of holes in the end plate of the internal baffle support tube which improved things considerably. I've never attempted to push past 90 but it now gets there easily enough and seems willing to keep revving.


1937 Rudge Ulster (project)
1946 Velocette MSS
1955 Triton
1959 Velocette Venom
1966 Triumph T120
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Originally Posted by Bry
turned out to be the repro fishtail silencer restricting performance.......got an extended drill bit and drilled a series of holes in the end plate of the internal baffle support tube which improved things considerably.
This suggestion is well worth looking into. I had to do the same thing to a pattern BSA BB34GS silencer some years back. It came with a very restrictive 5/8" (.625") hole through the baffle plate. I cut this out with a 1" hole saw from both ends completely removing the section of perforated tubing with the tiny through passage. Surprisingly, the end result wasn't that loud and the bike runs great.

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And Stuart, I bet your Goldie sounded EVEN BETTER on the 'twitter' even if not that much louder.

One day back in the dim mists of time my Venom suddenly went 'off-song' powering up the hill to my place.
I was convinced it'd seized or dropped a valve or something awful. Turned out it was just the 'secondary' rear fixed baffle in the fishtail that had self-destructed.
That was a relief!


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