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| Moderated by: Greg Fletcher |
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| Ignition Timing Values | Rating: 
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| Posted: 10-13-2005 09:13 am |
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1st Post |
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DanHolmes Member
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Hi, I have read a lot about ignition timing on these boards and most people are quoting figures of 8 or 12 degrees BTDC. I checked my vehicle (JH5 #18952 with twin Dellortos) with a strobe and it was showing 12 degrees BTDC and it runs very badly. A local mechanic told me that he thought the timing for my specific chassis number should actually be 2 degrees BTDC - a HUGE difference. Can anyone confirm this? Many thanks Dan
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| Posted: 10-13-2005 10:35 am |
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2nd Post |
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Harkes Member 
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2 degrees can't be right. 8 degrees static ignition timing is mostly used for emission controlled engines, so the 907 with strombergs. 12 degrees is the static ignition timing for 907 with dellortos. However these figures are based on engines with solid compression and correct cam timing. If your engine with dellortos doesn't run well with 12 degrees static timing than i would bring it down to 10 or further to 8. If it still doesn't run correctly, i would check compression and cam timing My new 2.2L engine with dellorto's should run with 15 degrees static timing and full advance of 28 degrees at 3500 rpm good luck erik
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| Posted: 10-13-2005 11:28 am |
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3rd Post |
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DanHolmes Member
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I have just checked with the mechanic who has looked at his old bit of greasy paper!! For Dellorto carbs - Ignition timing for chassis #'s prior to 18900 is 12deg BTDC but for chassis #'s AFTER 18900 it goes to 2deg BTDC. If this is true, what happened after chassis number 18900 to warrant such a big change?? Cheers
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| Posted: 10-13-2005 04:37 pm |
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4th Post |
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Mark Rosenbaum Member 
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It's my understanding that the various timing changes were the factory's response to the changes in emissions specs (primarily in the US) for the 1973, 1974, and 1975 model or calendar years (depending on how various local laws were written). Do recall that the level of understanding regarding exhaust emissions was far less back then. For the record, a careful reading of the shop manual along with other factory documents gives me the following values for the static timing on the JH and GT: Cars with Del'lortos: 1972: 12°BTDC, all markets 1973: 12°BTDC, all markets 1974: 12°BTDC, all markets (chassis to 18900) 1975-up: 02°BTDC, all markets (chassis 18900 up) Cars with Strombergs: 1972: 10°BTDC, all markets 1973: 10°BTDC, all markets (engines to 4803) 1974: 08°BTDC, all markets (engines 4803 up, chassis to 18900) 1975-up: 08°BTDC, North America and Japan (chassis 18900 up) 1975-up: 05°BTDC, Europe and Australia (chassis 18900 up) To further confuse the issue, the cam timing changed from 115° MOP to 110° MOP at engine 4030, and again to 110°/100° MOP at engine 10480 (for North American cars only). Additionally, it appears that many (not all) 1975-up cars also had the air injection pump, even when not required for a particular market, and it's my understanding that the final cam timing worked less than abysmally only with these air injection engines. IMHO the only reason to pay any attention to any of these numbers is when one must comply with regulations that involve ensuring that settings remain at the factory values (whether or not those values actually do result in low emissions). When that is not the case, one should use whatever settings give the best overall engine performance. For a stock low-compression 2 liter 907 engine with the 'C' cam -- the vast majority of JHs -- this happens to be with a 110° MOP and ignition timing 10° to 12° BTDC regardless of carburetors used. If, for whatever reason, it develops that an individual car happens to run better with different settings, use them instead -- nobody1 will mind. About the only thing one really needs to be careful of is to ensure that the ignition spark never occurs earlier than 38° to 40° BTDC at high engine speeds (again, for stock engines), as the engine will likely ping ('pink') or even knock (detonate) if the timing becomes further advanced. Fuel octane, type of ignition system, spark plug brand and heat range, cam selection, intake and exhaust porting, supercharging, and mechanical compression ratio, will all affect this limit, so again, different settings may apply to individual cars. 1. Smog nazis excluded, of course.
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| Posted: 10-13-2005 05:37 pm |
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5th Post |
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DanHolmes Member
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Mark, Many thanks for the clarification. As usual your depth of knowledge and precision saves the day!! I had problems getting the car through our UK MoT (Motor order test) on its emissions - it seems that the timing was set to 12deg but to get it to run at anything like normal the owner ran it very rich resulting in a lot of spitting and lousy fuel consumption (c.15mpg!). I weakened the mixture on the carbs 'manually' but had to retard the timing to 8deg to get it to run and pass the MoT. This has made it much better and cured the spitting problem but she is still running a little lumpy. I have some leave next week so am going to start from scratch with the dizzy - check points, dwell angle etc. and then retard the ignition a touch more to get it running a little smoother. If I can afford it I shall then put her on a rolling road for a proper tune up and 'sort-out'! Dan
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| Posted: 10-13-2005 08:43 pm |
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6th Post |
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Mark Rosenbaum Member
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Dan, A bit lumpy is normal -- the stock cams aren't all that high lift but the duration is 272° seat to seat, which by most standards is fairly radical. But the engine should smooth out fairly quickly. The advertisements used to say that a 907 engine would pull smoothly from 2000 rpm up, and in my car's case at least, that's true. It's my understanding that a JH in decent tune normally delivers between 20 and 28 miles per US gallon, depending primarily on how it's driven. I've never bothered to check what my car gets, but on a particular 20-mile mixed city/highway trip that I make occasionally, it seems to consume a bit less than a US gallon. I'm quite surprised to hear that you had problems with a static timing of 12°BTDC. Is there anything out of the ordinary about your engine? You might wish to verify that you have the correct later style crankshaft pulley and front cover, as using the wrong mix of parts apparently gives a 10° error between indicated and actual top dead center and therefore in ignition timing. A couple of owners have had this problem over the years. Fig. RM3 in the shop manual shows the two pulley versions, but the Front Cover line drawings in the parts book give a much better idea of what the front covers actually look like. It's my understanding that your car should have a type 23D4 distributor with Lucas part number 41488. There are a lot of distributors that will fit, but that's the only one with the correct centrifugal advance curve. Again, this is something to check when an engine shows unusual behaviour. What sort of emissions tests are they doing in the UK nowadays? Do they provide the owner with actual test data, or just a 'pass-fail' assessment? With good test data one can often tell a lot about an engine's condition and state of tune. If you do get a chance to put the car on a 'rolling road' I'd be very interested in hearing about the test, and the results. Last edited on 10-13-2005 08:47 pm by Mark Rosenbaum |
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| Posted: 03-27-2006 01:09 pm |
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7th Post |
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Jon Plowe Member
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Hi, I would agree that it depends on wear in parts, the correct ignition parts, carbs, head work, fuel rating. It's not unknown to get an engine with a mix and match of lotus and jensen spec 907 parts. I had my JH set up on a rolling road and the advance that gave the best power was not as per the factory, from my addled memory it was 14 degrees. So I would recommend this route (albeit more expensive) if you want to get the most power out on an engine. May also tell you other things about the state of your engine - is that always good thing?? One thing I found out was that was running a non ballasted coil on a system that had been converted to a ballasted coil - plenty of hard to find misfires and poor performance cured. I should have realised when the coil got so hot you couldn't touch it - doh!! Jon
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