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Ignition timing
Good morning all,
I’m hoping some of the guru’s on here can help with the above on my car, it’s an Amazon that during its life has had a B20E engine fitted converted to twin SU carbs, I have recently switched to DCOE’s and been on a rolling road dyno, the guy running the dyno has told me my timing is all over the place in that if he sets the idle timing to 10 degrees and then when he measures the max advance it is only advancing by an additional 14 degrees! Then when re-checking the timing at idle it has moved to 4 degrees! So what can cause this and is it possible that the B20E distributor (Bosch 0 231 163 021 JFUR4X) only has mechanical advance capability of 14 degrees and/or have I got another fault causing this, I can’t see how wear could cause the change in base ignition timing settings? I have removed the distributor and stripped it to check that it’s operating correctly and nothing is seized or appears out of place, is there anything I should be paying particular attention to? It’s been converted to Petronix electronic ignition and has the vacuum disconnected which I believe is vacuum retard on this unit. I was told by the PO that the car has a high lift cam but I’ve not been able to identify it’s exact spec, I also have documentation that says the cam has been set up with a dial gauge suggesting it has a vernier pulley, my question here is could I have a worn cam gear causing these symptoms? All comments and advice greatly received. Doug. |
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I don't know about this particular distributor, but I wonder why the vacuum advance (I'm think it will be an advance, why would one to retard the ignition with a larger induction vacuum?) has been removed. Why not try re-connecting the vacuum pipe to see if that helps, it may give some more advance? Alan PS. I'm guessing the Petronix electronic ignition is something like I have fitted to the Barge (240): https://i.imgur.com/HX6TEsG.jpg ... if so that just replaces the points and won't otherwise affect the ignition timing. |
Can you borrow a 123 dizzy to rule out the dizzy is the issue? It’s the first thing Amazon cars do when fault finding a ignition issue, put on the 123 dizzy that they know works.
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Hi Alan,
Yes that is a very similar variant of what is in my distributor and I believe that this specific distributor did actually have vacuum retard to try and help the manufacturers to comply with US emissions in the seventies, also I currently don’t have a vacuum take off anywhere on the inlet system! Mitch, Yes that would rule out my distributor but I’d like to try and understand what I currently have, however if I get no joy I may resort to replacing it with a known good unit! Doug |
Have you checked for side play in the rotor shaft, this can give erratic timing if the bushes are worn, if you can stretch to a 123 with programmable ignition do it especially as your changing carbs and have a performance cam, the original curve won’t be optimum, there is a basic data log option in the 123 that lets you see if your changes are improving the performance, easy to install and probably cheaper than taking it back to your mechanic in the long run.
I believe Amazon cars sell one with their own curve already installed but not sure if that’s the programmable one |
Hi Alan,
Yes that is a very similar variant of what is in my distributor and I believe that this specific distributor did actually have vacuum retard to try and help the manufacturers to comply with US emissions in the seventies, also I currently don’t have a vacuum take off anywhere on the inlet system! Mitch, Yes that would rule out my distributor but I’d like to try and understand what I currently have, however if I get no joy I may resort to replacing it with a known good unit! Doug Oops just spotted I managed to submit this twice!🤪 |
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Doug |
I have seen the original B20E distributor listed as JFURX 4 (what my Volvo SM shows) and JFUR4X. I have a B20E; but, ditched the distributor some time ago so I can't check the marking. Either way, I am guessing it is the same distributor that we are talking about.
The JFURx 4 is a vacuum retard distributor. It provides about 25 deg (crankshaft) mechanical advance and about -10 deg vacuum retard. If the ignition is set at 10 deg BTDC using a static method (engine off) and you start the engine the vacuum retard will pull the timing back to around 3-4 deg BTDC. The mechanical advance should achieve full advance by 3000 RPM (crankshaft). However, if you just rev the engine up to 3000 RPM the actual engine load remains low and the manifold vacuum actually increases (compared to idle) so you get more vacuum retard. If you are doing your tests with the vacuum servo attached, 4 deg BTDC at idle and 14 deg BTDC with the engine revved to 3000 RPM would be consistent with correct operation of the distributor. The ignition timing will only get to around 35 BTDC when the engine is at full load (vacuum retard has dropped to 0 deg). If you want to check the operation of the distributor, block off the vacuum retard, set the idle speed to 900 RPM and set the ignition to 10 Deg BTDC on the running engine. When you adjust the ignition the idle speed will likely change which means that you have to set it back to 900 RPM and re adjust the ignition timing. This will likely be an iterative process. With the vacuum retard blocked off, rev the engine to 3000 RPM and you should measure a timing of around 35 deg BTDC. This will likely be excessive and you will need to adjust the timing slightly wh.ch then alters the idle advance and the idle RPM. If the DCOEs will not idle at 900 RPM then you can't really use this process. Use a static method to set the timing to 10 deg BTDC. With the vacuum retard blocked off, rev the engine to 3000 RPM and set the timing to around 32 - 35 BTDC. As an observation, most B20 engines will operate satisfactorily on just mechanical advance (vacuum retard disconnected). The vacuum retard function on the B20E was a rather crude emission reduction strategy. Regardless of which method you use, with the vacuum advance blocked off you should achieve a maximum timing of about 32- 35 deg BTDC at around 3000 RPM. If you don't, then your mechanical advance is knackered. You can try a complete rebuild or just go for a 123 distributor. Edit - I read your follow up reply that implies that you are not using the vacuum retard. Your results suggest that the mechanical advance is knackered. However, be aware that ignition timing alters idle speed. With the vacuum retard blocked off, rev the engine to 3000 RPM and set the timing to around 32 - 35 BTDC and then check to see what the timing is at idle. If the idle speed is around 900 RPM the difference should be around 25 deg. If the engine will not idle at 900 RPM then the drop in advance will be different. The running idle advance is a function of the engine idle speed and 10 BTDC only really applies if the engine speed is close to 900 RPM. "I was told by the PO that the car has a high lift cam but I’ve not been able to identify it’s exact spec, I also have documentation that says the cam has been set up with a dial gauge suggesting it has a vernier pulley, my question here is could I have a worn cam gear causing these symptoms?" I think that is some hyperbole around the use of the word 'set'. When I rebuilt my B 20E, I used a dial gauge to confirm the open and close timing events of the intake and exhaust valves and their lift. This spec is provided in the Volvo green book. I suspect that you do not have a vernier adjustable timing gear. If you are brave, you can adjust the valve timing on the B20 by two methods. The first is to fabricate an offset woodruff key like this https://www.scootertuning.ca/en/cran...formances.html There is nothing 'adjustable' about the offset key. You get whatever is built into the offset. You do use the dial gauge method to confirm the offset in the key. I seem to recall that somebody was selling these offset keys for the B20. The second method is to just ditch the woodruff key completely in which case you can position the timing gear where ever you want and you truly are setting the valve timing by checking the valve opening events with a dial gauge. If the Volvo cam had a taper on the mounting end for the gear I would have less of a problem with this method; however, as I recall the mounting surface for the cam gear is pretty right angle so I would not be keen about trying this. You can use a dial gauge to determine the open and close timing events and the lift of the cam. You can then check against that B20 cam superlist to try and figure out what you might have. |
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Alan |
Wow thanks 142guy, when you say my mechanical advance might be knackered exactly what is knackered, it all moves freely and springs back when manually operated so what constitutes as knackered unit?
Doug |
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