suffolk moose Posted July 30, 2014 Report Share Posted July 30, 2014 Hi Guys, I'm looking for some collective wisdom! I have rebuilt my TR6 over the passed 25 years, and only just got it on the road this summer. It is /was ex USA spec. Because I have spent so long building it - my requirements have changed, I'm now older... And don't want a highly tuned car, just a reliable fun toy! My problem is - I may have put the wrong cam in 10 years ago... So looking for advice. The engine still has the low compression small valve head, I have a later higher compression unit, haven't fitted it yet, because I wanted to run the car in lower tune for a bit. I have fitted twin SUs from a saloon, BDM needles, electronic distributor, and the cam which is a Piper 285.( high lift stage3 - ultimate road) My problem is the car won't pull over about 4000rpm, just lots of pops and bangs. It will rev static to about 5500. I'm happy to put the big valve head on, if that cures the problem, but I'm not convinced it will. The timing is running about 30degree BTDC when fully advanced, and valve springs are matched to the cam - so no binding. Any advice where to start? Is it just over cammed? I had my doubts when I bought the piper at an international meet years ago - but it was cheap! Is the big valve head likely to cure it? Should I just go back and check the cam timing again? The car starts on the button, ticks over pretty well, slight lope, and is totally drivable in town, just has no top end! Thanks for any advice Pete Quote Link to post Share on other sites
Mk1PI Posted July 31, 2014 Report Share Posted July 31, 2014 Possibly going very lean? No reason that it shouldn't be tuneable with SU's and that cam - suggest a dyno tune with an SU specialist if all obvious things are OK. What air box are you using? Original saloon HS6 air box is very restrictive at higher flows. Quote Link to post Share on other sites
Tim D. Posted July 31, 2014 Report Share Posted July 31, 2014 Wilshires garage near cambridge know SUs well and did a great job on my spitfire some years ago. Quote Link to post Share on other sites
Peter Cobbold Posted July 31, 2014 Report Share Posted July 31, 2014 (edited) Welcome to the forum Pete. And congratuations on your persistence - 25 years! That low compression USA head will take a lot more advance so try 16-18BTDC static and see if it will pull more rpm under load. Peter Edited July 31, 2014 by Peter Cobbold Quote Link to post Share on other sites
poolboy Posted July 31, 2014 Report Share Posted July 31, 2014 You mentioned "electronic distributor". It would be worthwhile checking the rotor phasing. In other words, the rotor's alignment with the contact posts in the distributor cap. What might start out aligned at 'tickover' may change as the centrifugal advance weights rotate the rotor at higher rpms. Quote Link to post Share on other sites
suffolk moose Posted July 31, 2014 Author Report Share Posted July 31, 2014 All Thankyou for your interest - and your welcome! I am happy to go for dyno tune up, but realise that's a waste of money if the mechanicals are just wrong. If I can just supply a little more info? The SUs are jetted /needles as recommended by Piper, I can look up what I fitted. The SUs themselves are pretty good, spindles rebushed etc. Air filters are KN type pancakes ( I'm working on a cold air box, but these things take a while...). The timing is currently 18 BTDC at tickover - moves to about 35-40 BTDC when revved. I think im about as far advanced as I can get without any coughing back. Plugs all look really nice colour, exhaust smells slightly rich. Don't seem to get any pinking. Exhaust manifold is standard, stainless twin back boxes. I guess what I'm after is advice as to whether changing the head for larger exhaust valves will help? I will absolutely check the cam timing when I do it.... Or is the cam just wrong for a low comp head? Or is there something else I should be looking at. ? Thanks again Pete Quote Link to post Share on other sites
Mk1PI Posted July 31, 2014 Report Share Posted July 31, 2014 What CR do you think you have? 8.5 :1 ( head height approx 3.475" ) is not going to help the cam but it should still pull 5500-6000 cleanly. Quote Link to post Share on other sites
stuart Posted July 31, 2014 Report Share Posted July 31, 2014 You mentioned "electronic distributor". It would be worthwhile checking the rotor phasing. In other words, the rotor's alignment with the contact posts in the distributor cap. What might start out aligned at 'tickover' may change as the centrifugal advance weights rotate the rotor at higher rpms. Or if its one of those Chinese "Electronic distributors" they go so far off time as the revs increase you wont get anywhere with it. Stuart. Quote Link to post Share on other sites
suffolk moose Posted July 31, 2014 Author Report Share Posted July 31, 2014 The electronic distributor is from Power Spark, I had assumed them to be ok. The symptoms were similar though worse with the original mechanical version which is why I changed it, so I don't think there is any issue there. I believe the new one gives me the ability to open the engine up to more or less red line on the drive, which I never could before. I will check my notes on the head CR and depth when I get home, but it was at best 8.5:1. Thanks Pete Quote Link to post Share on other sites
Andy Moltu Posted July 31, 2014 Report Share Posted July 31, 2014 Have you got the original distributor? Swap back to that and see how things are. A rolling road tune can prove enlightening. It is possible cam timing is out - it's simple enough to check if you can borrow a timing protractor no 1 inlet should be fully open around 105 - 110 degrees after tdc (whilst the exact timing for a given after market cam varies slightly so long as it's in this ball park you can largely exclude cam timing as the cause of the problem) It's probably going to be fuel related and a rolling road session can be the best way to optimise the fuel and ignition settings. Quote Link to post Share on other sites
oldtuckunder Posted July 31, 2014 Report Share Posted July 31, 2014 My problem is the car won't pull over about 4000rpm, just lots of pops and bangs. It will rev static to about 5500. <snip> The car starts on the button, ticks over pretty well, slight lope, and is totally drivable in town, just has no top end! Thanks for any advice Pete As Mk1PI said even with a LC head 5500/6000 should be fine. I assume when you say it will rev static to 5500 you mean that it runs cleanly up to that RPM i.e with no load, but that under load over 4000 you get pops and bangs. Does that RPM point change depending on load, i.e. up a steep hill does the problem appear at a lower RPM than on the flat? I ask as when static even at 5500 RPM the air slides will be way lower than when the engine is under any load, and thus pulling way less air. I noticed you said you had fitted " Air filters are KN type pancakes" As late last year I was struggling with a loss of power when trying to accelerate hard on hills which was finally traced to **** pancake filters. I noticed your addition of the word "type" which means they may not be K&N's correctly sized for the engine. So if it was me and given what you say about starting, idling, driving in town, plug colour etc. before I started pulling everything apart I'd start by trying some simple experiments. 1) Remove the air filters, and try it, does it make any difference? 2) You say electronic distributor, do you mean you have replaced the points with a magnetic/optical trigger unit or something more radical? if its a points replacement unit, just pop in a set of original points and a new rotor arm, and really check where the rotor is pointing in respect to the cap at your static timing point, if its nearly past the cap conductor at static you could be jumping to the next conductor at higher revs. Look for the simple things first after a rebuild Alan Quote Link to post Share on other sites
oldtuckunder Posted July 31, 2014 Report Share Posted July 31, 2014 Sorry I see you answered the distributor question whilst I was typing, so as the problem is almost identical with both you can probably skip swoping back to points, but I'd still check the position of the rotor arm. Quote Link to post Share on other sites
Peter Cobbold Posted July 31, 2014 Report Share Posted July 31, 2014 (edited) Pancakes starving the engine of air. 150hp needs filters that flow about 105cfm (i.e. 55each) with 'no' pressure drop. Do a run wittout them, but avoid dusty roads. Edited July 31, 2014 by Peter Cobbold Quote Link to post Share on other sites
dingle Posted July 31, 2014 Report Share Posted July 31, 2014 Just to muddy the waters, I can see several possible culprits: The BDM needles are way too lean, especially with a performance cam. The CR would also be too low to be compatible with the cam. . The intake port spacing is different on the early carb engines (.640" for the early engines vs .840" for later ones). The appropriate intake manifold has to be used or the ports won't match. The early manifolds are log shaped whereas the later ones are more curved. Berry Quote Link to post Share on other sites
suffolk moose Posted August 1, 2014 Author Report Share Posted August 1, 2014 Thanks all for these wise words. Peter, Im away for the weekend, but will try a run without the filters early next week and report back. I did get them from moss about 10 years ago, hoping they would be up to the job, but quite possibly not! As I say, I am looking at get an cold air fed collection box sorted, as engine bay heat is going to be an issue - but get the engine to run properly first! Berry, the inlet port issue is something I may have known about, but overlooked. The manifold is from a 2500s, and the head is from a 74 US spec car, I will cross check the part numbers and report, but again - thanks its a good point. The replacement higher CR lead free head I have should match the manifold hopefully, but I will check. The car has been assembled over the years, and I simply couldn't afford the new head when I rebuilt the engine, which is why it's not installed, but if air flow/CR is the source of the problems - it can soon be fitted!!! Equally if it's a carb set up issue, again I have a target to go for, and that would be worth taking to a specialist if i really can't sort it. Cheers Pete Quote Link to post Share on other sites
Peter Cobbold Posted August 1, 2014 Report Share Posted August 1, 2014 It could also lean out badly at higher outputs due to poor fuel supply. What pump? fuel filter? what is feed pressure to SU set to? Quote Link to post Share on other sites
njc Posted August 4, 2014 Report Share Posted August 4, 2014 (edited) Could be coil voltage. >4000rpm it starts to reduce in output. Try standard points and wire coil direct to 12 volt (assuming 12 volt coil) I had similar issue with electronic ignition and balasted coil plus tired wiring. When revving above 4000 rpm power faded Edited August 4, 2014 by njc Quote Link to post Share on other sites
suffolk moose Posted August 7, 2014 Author Report Share Posted August 7, 2014 Just an update of yesterday's investigations! The cylinder head is a late carb model, I'm just about certain there is no mismatch in manifold/ports. I it's a few years ago since I built it - but my eyesight was better - and I would have spotted it I'm sure..... CR should be 8.5:1. Low for the cam - but I would still have expected more revs, although not necessarily power. I went for a run without the air filters - no difference at all in performance. Again the engine will red line on the drive, but just hits a wall at about 4000 rpm in the gears. I replaced air filters, and removed vacuum advance tube, which did appear to make a slight difference. Pulled the needles from the cards, and they are only BDQs -It must have seemed a good idea at the time! I now have a pair of BAE on order - will update when these are fitted. Im sure they will do something, and this will then indicate whether I need to look deeper into electrics or fuel pressure. Pete Quote Link to post Share on other sites
suffolk moose Posted August 26, 2014 Author Report Share Posted August 26, 2014 Hopefully I can now put this topic to bed, Thankyou to everyone who showed an interest, and offered advice. The bottom line to the problem was the SU needles being far too lean at the top end. I installed the BAE needles, and straight out of the drive noticed the difference, and when warmed up the engine would easily spin up to the red line through the gears. A totally different car, and what a fantastic noise!! So just a bit of fine tuning now, hoping we get some more sunny days this year, then come the autumn I will put the higher CR head on it, and see what that brings. Thanks again, and will see you all on the next challenging topic. Pete Quote Link to post Share on other sites
dingle Posted August 27, 2014 Report Share Posted August 27, 2014 I think you will need even richer needles from mid-range than the BAE. If the head you are using is from a 74 carb car, it was originally 7.5 or 7.75 CR. , When the higher CR head (9.5) is fitted, the dist. should be recurved to match the one used on the PI cars and the static timing reset to about 12 deg. Berry Quote Link to post Share on other sites
John L Posted August 27, 2014 Report Share Posted August 27, 2014 Here is a useful chart to compare needle sizes, Hope it might be of use. http://www.mintylamb.co.uk/suneedle/ John Quote Link to post Share on other sites
Blue cedar Posted August 27, 2014 Report Share Posted August 27, 2014 Pete That is a really good outcome. Congratulations to the forum and it's paticipants. The forum has helped me too with 'front end rattle' ....brakes. I got the spring kit in the post today and look forward to fitting a testing this weekend, weather permitting. Best regards to everyone. Peter Quote Link to post Share on other sites
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.