Acceleration stops at 50mph - max speed=50mph

Engine Oily Bits, Ignition, Fuelling, Cooling, Exhaust, etc.
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Forkie
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Re: Acceleration stops at 50mph - max speed=50mph

#71 Post by Forkie » Sun Jan 19, 2025 6:08 pm

So the latest with my running issues - to re-cap i believe my HS6's are out of tune/balance. According to the spark plug colour, along with a Gunsons spark plug kit, both gave the indication that the front carb is a tad weak, the rear is rich. Made my own mixture screw spanner - but to even get onto the rear carb i have to remove the elephants trunk everytime - a bit of a pain but hey ho. So i turned the rear screw a half a flat anticlockwise to weaken it , the front a quarter turn clockwise to enrich. Then we waited for a dry day, yesterday, to just be able to start, and run the car normally rather than just let it idle in the garage.
Did the usual ' dry start' procedure, then started normally and all was well. Once on the road and choke now in she was not happy at all. Popping and banging as soon as throttled , way worse. Luckily i had left required spanners to adjust in the car. So reset to where it was, and drove home all ok.
The lad then suggested doing the same frugal adjustments, but in the opposite way - to me that felt wrong - as in that would enhance the issue, but i did - and we went back out. Way better, much smoother, until we hit an incline, again....power just drops away. On the level, or downhill all is good. When sat still at idle, still feels lumpy. Rev to 1500 - 2000 its really lumpy...
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Forkie
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Re: Acceleration stops at 50mph - max speed=50mph

#72 Post by Forkie » Sun Jan 19, 2025 6:28 pm

This afternoon i decided to just 'jump in and start' as it was used yesterday, so no dry start etc. So i just jumped in, pulled the choke fully out, no throttle, and turned the key. Within one revolution she fired and ran well - no issues. Normal !!!!! As it warmed up/ choke going in thats when the lumpy/ uneveness started - uneven idle, but not even on the miss side - random - when putting foot on throttle to say 1500 - 1800 rpm it was lumpy - can feel it through the car, hear it through the exhaust . It was almost like firing on 5.5 cylinders.
New exhaust, coil, battery, plugs, leads, rotor arm, dist cap, connections all checked, numerous times.Oil in dashpots. All new gaskets from head outwards. Compression tested all 151 x6 cylinders. No overheating issues. Always stays bang on center ( new water pump as well). I am now thinking a full carb strip down - it is one thing i have never done....
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1975 TRIUMPH 2500S AUTO ESTATE IN BRG!

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Forkie
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Re: Acceleration stops at 50mph - max speed=50mph

#73 Post by Forkie » Sun Jan 19, 2025 7:33 pm

20250118_160256.jpg
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Re: Acceleration stops at 50mph - max speed=50mph

#74 Post by Forkie » Sun Jan 19, 2025 7:34 pm

20250119_165221.jpg
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Re: Acceleration stops at 50mph - max speed=50mph

#75 Post by Forkie » Sun Jan 19, 2025 7:41 pm

First pic is of number 5, after turning mixture screw of rear carb anti clockwise, apparently to weaken mixture. Engine performing worse than previously. Second pic - sorry for duplicate - after reinstating to how it was, but now turning half a flat the other way to enrich - clockwise - engine now slightly smoother on flat roads, or downhill - but still not liking an incline/ lumpy idle etc
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DON'T TIDY UP - LEAVE IT WHERE IT FELL!!

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Re: Acceleration stops at 50mph - max speed=50mph

#76 Post by Forkie » Sun Jan 19, 2025 7:44 pm

First pic is of number 5, after turning mixture screw of rear carb anti clockwise, apparently to weaken mixture. Engine performing worse than previously. Second pic - sorry for duplicate - after reinstating to how it was, but now turning half a flat the other way to enrich - clockwise - engine now slightly smoother on flat roads, or downhill - but still not liking an incline/ lumpy idle etc
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Re: Acceleration stops at 50mph - max speed=50mph

#77 Post by johnnydog » Sun Jan 19, 2025 10:14 pm

An uneven and irregular engine beat is usually a sign of a weak mixture - a regular hunting is usually associated with a rich mixture.
Assuming the air flow to each carb is balanced and the throttle linkage is operating simultaneously on both, then I'd whip the pistons out of each carb and check the height of each jet. In my experience a good starting point is each jet being approx. 1mm below the jet body and the bridge. Once that is equal, some minor adjustments to get a nice smooth idle at tick over (if a regular hunting then too rich; if a irregular beat then it's too weak). You can do these adjustments without replacing the elephants trunk every time and initially adjust both carbs equally. If you are making regular tweaks to the mixture, it's worth noting down how much you have altered them as it's easy to loose track of how much you have adjusted them! You should be able to increase the revs, and then release the throttle (whether by hand or foot!) and the revs should fall to a constant tickover without any stuttering. If you get these right, it won't be too far out.
I don't really know what else to suggest other than someone possibly in the Register who is willing to have a play with them!
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Re: Acceleration stops at 50mph - max speed=50mph

#78 Post by Clifford Pope » Mon Jan 20, 2025 11:34 am

Forkie wrote:
Sun Jan 19, 2025 7:44 pm
after turning mixture screw of rear carb anti clockwise, apparently to weaken mixture.
As someone has said before - it's upside down. Anticlockwise, as looked at from underneath, lowers the jet seating, so enrichening the mixture. (It's like a tap in a length of hose - it still opens anticlockwise whatever its orientation !)

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Re: Acceleration stops at 50mph - max speed=50mph

#79 Post by Forkie » Tue Jan 21, 2025 1:19 pm

Yes i have taken note of the small movements that i have made. I think i am going to have them off and purchase rebuild kits and needles, at least then i know it has been done - everything else has had the treatment so why not :|
Having been on the SU website i did not realise there were so many different kits, even for the same carb types - part numbers i have come across so far are CRK 149, CRK 296, CRK 249, CRK 257 to name a few - all for pairs - using there site checker it comes up as CRK 149 - is there a way of checking that? Also, same with the needles - they do not come with the rebuild kit, so a seperate purchase - again where do i check for the correct ones? I have not rung them to check yet, thought i would ask here first.
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Re: Acceleration stops at 50mph - max speed=50mph

#80 Post by johnnydog » Wed Jan 22, 2025 9:20 am

Without going back through all the posts, have your HS6's got waxstats or red tipped jets? If it has got waxstats, then I would definately consider changing them to the red tip jets. Red tip HS6's seem much more reliable and are easier to work with.
Have your throttle discs (butterflies) got the poppet valves on them? Poppet valves are circular with a spring on one side and can be seen on the flat face of the throttle disc. Something to do with emissions I believe. I was advised by an 'old school' tuning chap many years ago that they can reduce the fuel / air flow, and his suggestion was to get rid of them and use the plain flat throttle discs on each carb.
I felt my S was a little lethargic at higher revs (not that I drive it hard) but I noticied it was a little flat at the top end. In answer to your q's about needles, the standard needles are BDB. I posted on here a while ago about my search for a suitable alternative needle that gradually enriched the fuel slightly higher up the rev range. BPZ or BDM needles give that gradual increase in fuel delivery up the rev range - I settled for BDM in mine and noticied an immediate improvement in top end acceleration, whilst retaining the correct idle speed mixture. Dave B has BPZ in his, although they were fitted when he was using a PI engine on carbs, although he now runs a 2500S engine in it with the same needles and it does run very well. The fuel delivery graphs for both run more or less along the same path - BDM are very slightly richer than the BPZ at to top end but the difference is negligible. Incidentally CW recommends a 2.5PI to use BPZ if on HS6's.
I also removed my throttle discs with poppet valves, and used the standard plain throttle discs.
If you search under the topic 'Re: cut-outs after reving above 4500rpm', post no.6 dated 28/12/23 gives all this info, plus the graph from Minty SU with the relative performance data of the three needles!
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1967 Mk1 2000 in Gunmetal Grey
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